Is there a downside to using multiple relays simultaneously to increase relay capacity

Unless you can guarantee that all the contacts will close and open at exactly the same instant of time the only safe current you can assume is 2A - that is, the capacity of the first contacts to close, or the last contacts to separate.

Sixty-four little relays were turned on by a key;
One soon welded shut and then there were sixty-three.

Sixty-three little relays started glowing bright blue;
One of them turned black and then there were sixty-two.

Sixty-two little relays bore of amps a metric ton;
One said he'd had enough and then there were sixty-one.

...

(I can't be bothered to write 64 stanzas. I hope you get the idea. It ends with your house catching fire.)

Here's what will happen. When you activate your relays for the first time, the first one to close will have its contacts welded together by a massive overcurrent. Soon enough, other relays will close and hopefully distribute that current more or less evenly.

When you deactivate your relays, they will all open except the one with welded contacts. Since it will now be alone under full 64A load, it will soon blow up.

Now you have 31 little relays standin' in a line.