Andy, there are crosstop levers designed for v-brakes, unfortunately not cheap unless you have a friend in the US, the options are:-
The Tektro RL740 which I've only ever found at Rivendell Bikes
http://www.rivbike.com/products/show/tektro-rl740-v-brake-levers/15-159 the price is fine but they have a flat rate of $50 for international shipping
Alternatively there's Pauls Engineering crosstop
http://www.paulcomp.com/crosslever.html which has a moveable pivot point and can work with canti or v-brakes, pricier but his shipping is cheaper, just got some other bits from him
Other options include just using a regular crosstop, some people claim it works fine, I haven't tried it but there have been instances of new bikes coming setup this way.
From personal experience I found two crosstops was overkill and reduced to just the one and for the front brake only. But it was only ever really for modulation when cruising on the top. Any serious braking was on to the hoods or the drops, by the way the RL520 are very good with v's.
If you ever do decide one crosstop is enough you could solve any issues by going to an Avid BB7 road version cable disc brake and a normal road lever with a normal crosstop, obviously forks need disc tab or you need new forks
