First build stairs to the island that you want to connect to. Make sure the stairs go far enough.
Then put some bricktons on the end of it.
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=208621849

Use another Brickton on the "home" island to destory the basis for the stairs, this will let the Bricktons fall down on the island you want to connect to.
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=208621977

With Bricktons on both sides you can now build up stairs that meet in the middle.
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=208622023

After that the two islands are connected.
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=208622068

This works only when the target island is large enough and the bricktons don't fall when you destory the stairs.
Best is you save before and if something goes wrong you just load and try again.