It seems that the "where can I make a hole" question has been answered.
"Are two U-Bends acceptable?" - not so much.
I think the answer is no, unless you can fit the sort which has an integral removable cleanout plug and can provide access to said plug. One u-bend (trap) is needed to put a barrier of standing water between you inside your house and the sewer gases lurking beyond. However, from time to time that one can become obstructed and not drain, and has to be accessed for removal of whatever didn't pass through. Having two traps in succession means the far one will be hard to reach for cleaning. AND, downstream from the trap (each trap) you need a vent pipe connection, or fluids passing through the pipe on their way to the sewer main may 'suck' the water out of one of the traps, breaking the seal against those gases.
So, unless you can get a plumbing expert to sign off, two traps in the same waste line is a no-no.
If, however, you've got several fixtures draining into a common sewer pipe (one that's got a vent on the end of it), no problem with each having its own trap / u-bend as long as the distance from the fixture to the main pipe is reasonably short. That's the way it's supposed to be - sink/lavatory has a trap, shower has a trap on its drain line, etc.