Dual 7731 in Link Agg mode

I'd say yes, but you'll need more than just 2 pair of 7731s. You'll also require a switch on both sides of the link to handle load-balancing and/or aggregation.

This is correct. Link aggregation is handled by intelligent switches (see LACP). In this sense, a pair of 7731 is just one link between two switches.

I would be more concerned about interference between the two pairs. Even when you choose lowest and highest channels. Does anybody have experience with such a configurartion? How far apart should the 7731s be at either end? 50cm, 1m, 5m?

At minimum I would put them 1m apart and at least 1 channel empty in between.

Thanks for the info, but i changed my design, so i don't need this setup.

I tried. One pair will in disable mode but failover is working if the active link is down. I test with peer to peer bandwidth test but the result showed the bandwidth not double up.

This is not surprising. The load balancing is done for end to end pairs, typically based on MAC addresses. When you measure between two machines, only one link will be used. Measure between a different pair of machines at the same time and you should see both getting the full bandwith of one link.

If you want to double the bandwith for a single communication pair, it gets much more complicated. You would need high speed routers on both ends to do packet based load balancing.