Are there any side effects when two distros share a swap partition?

It's possible. In fact, you can share the swap space between completely different operating systems, as long as you initialize the swap space when you boot. It used to be relatively common to share swap space between Linux and Windows, back when it represented a significant portion of your hard disk.

Two restrictions come to mind:

• The OSes cannot be running concurrently (which you might want to do with virtual machines).
• You can't hibernate one of the OSes while you run another.

One side effect I can think of is:

1. Hibernate system1 (using the swap partition for hibernation).
2. Boot system2.

You could lose data.

One of my friends tried this. He has installed five or six distributions in a single hard drive.

The first primary partition is for GRUB, and he is able to boot to all the distributions. The second partition is swap. The third partition is an extended partition and each of the distributions are installed into their own logical partitions.

All of the distributions are able to boot and can hibernate. I think you just need to make sure and select the correct distribution after resuming from hibernation.

So, on the basis of his experiment I should say YES this is possible, but I think it can break things. What if distribution 2 wakes up and distribution 1's resume file is using up the swap partition; what's the next thing that's going to happen?
So I, too, agree with all the above posts. Why don't you try to split the swap partitions, rather than taking this huge risk?