The two timer achievement is awarded when you successfully get Chloe to be your GF while already being in a relationship with Lauren. This achievement took me far too long to get, only making it on the 3rd attempt after carefully reviewing the game files. The correct process to unlock it isn't all that difficult, however it does require some specific choices and hence the purpose of this guide.

Two Timer
Be Involved with Chloe and Lauren

Timing
This achievement is unlocked during the car test drive with Chloe in London after the MC asks Chloe to be his GF.
Conditions

• Chloe is GF
  
• Relationship with Lauren
  
• Not had 'scene' with Aubrey on the plane.

The third one is pretty self explanatory. The other two I will go into more detail below. The general setup though is that it is basically a Chloe run with some additional steps for Lauren.

The achievement requires that Chloe becomes MC's GF during the test drive scene. This trigger has a few dependencies that I will trace back the conditions below.

Scene v11s34: Car Test Drive - Parked on street swapping drivers

• Kiss her
  
• Be my girlfriend

For this sequence to succeed you need

1. Relationship with Chloe (will have by default anyway if the other conditions are met)
   
2. Meet up with Chloe for a swim in Act 1
   
3. Take Chloe to homecoming
   
4. End homecoming with Chloe

Ending homecoming with Chloe and going back to her place will trigger this. So requirement 3 will give you this by default provided you don't make her mad in Act 2 or 3 (which AFAIK you can't actually do anything to make her mad after Act 1).

Scene Act 1, Day 6: In the park after Adam fight (or not) and Imre convo.

• Meet Chloe

Requires

• Chloe is not mad at MC

Scene Act 1, Day 10: In room, choosing hoco date.

• Choose Chloe

Requires

• Not currently in a relationship with Lauren
  
• Chloe is not mad at MC
  
• Either KCT Popular OR bought the volleyball when shopping with Julia

Scene Act 1: Homecoming freeroam.

• End homecoming with Chloe

Requires

• Either KCT Popular OR started hoco with Chloe. (for the purposes of this achievement, the latter will be met by default)

The achievement requires that there is a relationship between the MC and Lauren at the time when the MC asks Chloe to be his GF. AFAIK there are 4 moments when you can start a relationship with Lauren, two in Act 1 and one each in acts 2 & 3. For the purposes of this achievement starting a relationship in Act 1 is not recommended as you will need to break it off in order to take Chloe to homecoming. While it is possible to start the relationship, break it, restart it, and still get the achievement this guide will ignore that pathway.

There are multiple pathways to do this. The suggested approach here is sort of 'future proofed' in that it gives you more options for starting a relationship later.

Act 1 day 2: In the park with Lauren

• Yet, you're here with me
  
• I've broken into an Ikea
  
• Kiss her

This will give the 'Romeo' achievement

Then either turn down Lauren's meeting invitation or accept and tell Lauren to "Forget about it" in the meetup at the cafe.

There is the opportunity to start a relationship with Lauren in Act 2 when you meet up for a second time to discuss her fundraiser stall .
Scene v8s24: Lauren's room on bed
After the conversation during the make out session

• Suck on her tits

This sequence will start a relationship with Lauren if the following requirements are met

• KCT Loyal
  
• Kissed Lauren on park bench ('Romeo' Achievement) OR Kissed Lauren during beach 'date' OR Kissed Lauren during movie date (this is for if you started a relationship in Act 1 and later broke up)

Finally, don't have sex with Aubrey on the plane in Act 3 because this will end the relationship with Lauren.

The last chance to start a relationship with Lauren for the Two Timer achievement is at the end of the Hotter Parry trivia. Getting the questions right doesn't seem to affect the starting of a relationship later but they do grant "Lauren likes you" points so you might as well.

Scene v11s30: Hotter Parry Trivia with Lauren

• Friggindoor
  
• 29
  
• 12 Someold Place
  
• 7

Then later in the scene when Lauren will tell you that she loves you if the following requirements are met:

• KCT Loyal
  
• Kissed Lauren on park bench ('Romeo' Achievement) OR Kissed Lauren during beach 'date' in Act 1

Then when you reply

• I love you too

A relationship with Lauren will start

• Wolf or Ape? Doesn't matter.
  
• Because the pathway to getting Chloe to be your GF involves never making her mad or turning her down, you don't actually need KCT Popular at any point (if you buy the volleyball). Hence it should be easy to maintain KCT Loyal to start the relationship with Lauren.
  
• You don't need to be loyal to only Chloe and Lauren. In fact I was able to get this achievement on a 'body count' run.
  
• It is worth noting that Chloe has two 'levels' of relationship possible during Act 3 (well four if you count neutral and her being mad at you). That is: relationship with Chloe and GF/BF with Chloe. Both statuses will enable both the London and Amsterdam scenes, the difference is that the latter has a few alternate dialogues throughout the remainder of Act 3. Presumably there will be further differences in future acts as well.
  
• One of the dialogue changes as a consequence of this achievement is that Charli calls you out on this two-timing. So it would be wise to be very cautious of him going forward if you want to sustain both relationships. To this end I will speculate that not ratting Charli out could be useful for blackmail later.
  
• You might not need to start a new game to get this achievement. If you have a Chloe run save from the start of Act 3 that you also have gotten the 'Romeo' achievement and can get to KCT Loyal in time you will be able to pull this achievement off.

This is my first guide so any tips, questions or corrections are welcomed :)