Have a xbox problem where users are randomly able to access xbox live. It appears that once one xbox gets online and plays, the second is unable to communicate with xbox live.
I have set port triggering in the router and UPNP but this is not working.
How should I configure a mikrotik to allow the xbox hosts to get at 3074, 88 and 53?

Spending some time searching, I came across a post from 08 that speaks of this issue. Post link regarding a problem with UPNP and Xbox's was provided. Has their been a fix or work around since then?

That's impossible. Think about it. You're trying to forward tcp/88 and tcp and udp/3074 (port 53 on either protocol is irrelevant for XBox Live). If you were able to somehow forward the same port on the WAN side to multiple XBox devices on the LAN side, how would the router decide who to forward a packet to? Packets for all devices would come to the same port. There is absolutely no indication of which device the other party is actually trying to reach.

You need one public IP per XBox.

Mind you XBL will work without port forwarding just fine - you'll just be in the state XBL calls "moderate NAT", which may lead to longer latency when joining games and discovering other players. But you cannot forward the same port on the same IP to multiple inside machines.

It has been my experience that if you have multiple xboxs behind a firewall and with upnp enabled, each xbox will pick a port.

the first will pick 3074
the second will pick 6148

and so on. I've run 3 at a time.

I will throw a 750 at it tomorrow and see if MKT UPNP is any different from the router currently in place and see what I come up with.