I have 2 wAP ac mounted for a wireless link between two buildings. The link transports 5 vlans. Problem is that a few times a day some cars stop outside and the link drops. So I'm thinking, as the wAP ac has two radios, I could use them both, and hopefully when cars passes, only one of the frequencies are disturbed. So, how should I make use of the the dual radios to make the wireless link most stable, and preferably also increase speed while cars are not disturbing?

Ideally to do what you want, the radio would be connectorised so you can have the two sets of antennae some distance apart [so that when one is obscured, the other isn't, hopefully].

You haven't mentioned which of the two bands you're using at present. Are you able to repeat the obstruction on demand? If so, switch to the other band, obstruct the link and see if the link stays up. If it doesn't stay up on either band then there's no point in using diverse links [although it might be a fun learning exercise].

Is it possible to change this network to routed rather than bridged? If so you can get a bit more clever with it, eg full duplex:

http://blog.butchevans.com/2008/10/usin ... ess-links/
https://wiki.mikrotik.com/wiki/OSPF_to_ ... redundancy

I guess you could then bond a pair of EoIP tunnels over it if you really want L2.

I've tried both bands, different frequencies. They all seems to drop, but not at the same time. Currently 2GHz band is the one in use, and I have configured a test subnet for a 5GHz link. I have a ping running on the 5GHz link. From the log it seems like the 5GHz is still alive when the 2GHz disconnects. So if I could use both bands with a fast failover this would probably make a significant improvement to the uptime.

People are moving from one building to the other while they are in a skype meeting! They expect their devices to switch from AP in building1 to another AP in building2 while being connected. I don't think wlan can be configured to do this switch smoothly if the link is routed? Also I need the same vlans on both sides. Maybe EoIP tunnel could help to solve this over a routed connection? (I've never looked into EoIP)

What would happen if I just put the 2ghz and 5ghz radios into the same bridge and enabled (R)STP for the bridge? Could any logic be added so the bridge chose the network with the currently best capacity? Maybe a script checking current signal level, and setting priority to the connection with the best signal?

That's the "easy" answer but doesn't fulfil your original requirement of load balancing. Maybe bonding the two links would, however?

From reading the manual on bonding, bonding seems to be the way to go. Choosing the bonding mode seems to require some experimenting. And while the manual states that wireless interfaces can be bounded, the examples states that they can not unless a layer of EoIP is added. I'm a bit confused, but I think you've pointed me in the right direction Troffasky. Thank you.

Bonding two EoIP tunnels over two wlan links seems quite stable using broadcast as bond mode.
The only issue I ran into is that when a client moves from one site to the other, traffic stops for 30-60 seconds. I followed an example of bonding eoip where rstp was used on the bridge. I suspect the rstp to be the cause of the delay. I've now disabled rstp and will test if it got better the next time I'm onsite.