If presentation is not an issue, I'd carry a good quality swiss army knife; I routinely trim my beard with the built in scissors and they work well. It's useful for many other things too, though difficult to take on a plane

If you're after maintaining a neat beard or designer stubble look, it's hard to beat a battery powered beard trimmer for neatness, portability and utility - I use mine for plenty of other things, including hair and even removing scratchy labels from clothing. Trimming once a fortnight I doubt I'd get though more than a couple of sets of batteries

If you're after a life-hacky way to remove hair, you could consider burning it off with a quick pass of a gas flame - a high speed pass, as many juveniles have found when leaving the gas hob to run for too long before lighting it/filling their mouth with gas from a cigarette lighter before imitating a circus act, is effective at singeing hair to the extent that it goes brittle and can be brushed off, without burning the skin
Lastly, if you're really struggling for space it may be worth noting that hair cutting is pretty much a universal need for human beings and it's doubtful you'd travel for months on end without happening upon somewhere local that will remove your excess hair, or a shop that will provide you with something that can hack the worst of it off. Like so many things we think about when packing for travel in order to remain connected to the lifestyle we have at home, ask yourself whether it's truly needed at all, and if it really is - is there some reasonable amount of money (a highly portable and versatile travel accessory :) ) for which one can procure the relevant details locally rather than attempting to carry things to cater for every eventuality