You could add a simple 5-port managed switch at each end and run both over if one cable using VLANs (or move the Draytek and add a switch at the other end I guess).

All switched into Router mode now. Waiting for the VirginMedia line to fail so I can see if the failover works.

Hello Paul,
Just came across this thread and was more than a bit interested in your comment about DNS/name resolution failure.
I have a pair of 2925s in HA mode, but that's not the important bit here, the important thing is that I have dual WAN (BT Openreach & VIrgin BB via SH2 in modem mode.)
The BT line is absolutely ROCK SOLID, but the Virgin line sometimes "glitches", generally for less than a minute, but I noticed name resolution issues in some of the things I was doing on a PC when this happened.
Further investigation showed that the Draytek hadn't "failed over" the PC connection to the BT (working) interface, but if I did a test, at the 2925 by unplugging the SH2, it would correctly failover.
I played around with the options for link detection ('though I DIDN'T try the 'WAN ping" option) without success.
Eventually, I happened to be on the 2925 web interface when the Virgin line did it's "funny" and I think I can now explain why the failover didn't/doesn't work.
The SH2 had lost it's WAN IP address (for some reason) and when it does this, it defaults to a 192.168.100.1 (I think) IP address - no good for getting out on the internet, BUT, as far as the Draytek is concerned, it can still see the interface as "up" and therefore doesn't failover. Obviously, my manual "cable pull did trigger the failover 'cos it couldn't see the interface at all at that point.
So, whilst the WAN interface has the 192.168.100.x IP address, I have problems, once the WAN interface gets it's true IP address, things spring back into life.
Based on what I've found, I'd guess that the "WAN ping" is the only reliable test that the Draytek can base the failover decision on, but I haven't had a chance to try it.
I have found that there's a web site (thinkbroadband) that you can use as an external monitor - it will ping your WAN IP address and give you a graph of results - so I set this up and sure enough, I could see the Virgin interface was going unreachable, if only for very short periods of time.
Sorry this is a very long winded post and doesn't offer any answers, but I hope you might find it interesting.
All the best
Simon

Thanks Simon. After a lot of messing about, I came to very similar conclusions.

I know that local name resolution problems can occur if you are using VM's DNS servers because they respond to every request they can't resolve with a stream of advertising rather than the required error message. I quit using Virgin's DNS long ago because of that.