anyone know what kind of current a twin motor starter box draws
 

? as per the title, might sound a bit odd but Im taking about a starter box with 2 motors in it, and building a starter stand for my nitro bike

what I need to do is sort a switch for it, but no idea what kind of current rating it will need to be

I suppose at a push I could just use a big 70 or 100A relay?? with a smaller switch?

Relay's probably the neatest option or an engine cut switch from a car accessory shop (but end of the day your battery will be limited on output)
What battery are you using? Double the Ah and a switch that big will be fine I'm sure
I used to have a camper van conversion with 2 batteries, I had a 32A switch between the start circuit so I didn't use the leisure battery for starting, on a freezing cold snowy day the van would not start so I opened the switch and away it went!! (It was a good month before I bought a new main battery, the switch was fine)