The most important thing regarding bike to bike /truck / car or what ever, is not so much the power output, its the antenna. A small hand held walkie talkie with a "rubber bendy" antenna will have limited usefulness in any terrain. If you can fit an external antenna by way a coax cable to the body of your vehicle it will make all the difference. It must however be the correct antenna for your radio transmit/receive frequency. It would ideally need to be matched or tuned correctly to be most efficient.(not hard but equipment required for this - so ignore ) Pre tuned or match antennas are available and will be near enough in most cases. Most radios that people use tend to be in the 400-450mhz range which equates to quite a small antenna in reality not easily distinguishable from broadcast radio antenna, so pretty much "undercover". There is a set of frequencies used for exactly this use often referred to as 446PMR and many sets of walkie talkies are available for use across most of Europe and poss. some other countries. Google will help with that. The issue with most of these radios is that they are limited power - 500mw I think, and have NO external antenna socket. This is the legal acquirement as far as I as last aware. So to make best use of this system you would have to step outside of these rules. That would make you a bad person.

But say you did.....the radio would be small, the antenna would be small.....

Waters & Stanton Watson Mobile Antennas

This company sells antennas you would need "70cm" band or "2m / 70cm" band antenna to use on a PMR446mhz radio. They are not that long ! about 50cm in length on a magnetic base, cost about 20GBP(£). You may require an adapter to plug in at radio end. There are some 5watt walkietalkies on ebay for peanuts which work well, which have an external antenna socket which you unscrew the rubber antenna to get at. Some are refered to as VHF which need a "2m" type antenna The ones which are "UHF" require a "70cm" type antenna. There is lot more involved really but it can be simplified and still work far better than just two walkie talkies inside a car etc..

So basically get a decent external vehicle mounted antenna and you will be sorted.