2 cubes collide and fall in water, how to get one

You need to drop the cube while standing on the platform. The platform will be down while the cube is blocking the beam.

1. block beam with the cube (on top of hard light)
2. step on the platform
3. make cube drop by redirecting the hard light
4. beam will activate the platform, which will get you to the exit, cube will open exit

First tip, you're not supposed to go to that exit yet. Take a look around and try something else. You'll know when you're supposed to go up there.

Another hint:

You should get to the top floor of the office building, you'll find something inside

Solution:

Portal up to the platform like you did, and then put a portal on the sloped surface at the left of your second screenshot. Jump down from the platform and open a portal on the ground below you. This will catapult you to the upper floor of the office building

The next step after that:

Now the bridge should be extended and you should be able to jump down from the platform and use that momentum to get to the exit.