It is tough to make a living as doubles specialists if you are in challengers (ranks below #125 or so). You make an average of may be a $500-750 a week even if you are winning some 5-6 titles in the 20 odd challengers you play. Too little money to even pay for the airtickets.
You CAN make a living as just doubles specialists if you are in ATP tour events (rank inside top-60 or so for making almost all events, but somebody inside top-100 can find enough events to play). Make a minimum of $750 for even entering the event, and at least an average of $2000 per week to survive. There are probably about 20 or 30 players who do make a decent living like that now.
The real problem is that it has become EXTREMELY difficult to move from the challenger level to the tour level. The maximum doubles rank that you could get to, at the challenger level is about #125, say about 400 points .. You need about 4-5 titles in challengers (50 or 60 points each) and a bunch of finals to even get to #125. But from there to take the rank to say #80 (about 750 pts) is very tough and virtually impossible by playing challenger doubles. This problem became worse once ATP reduced the entry spots for doubles ranks (so, most cuts are at around #150 to #225 in sum of rank). You really need somebody in the top-75 take you on and sign in with you or somethimg - which means the challenger doubles specialist needs to show up at the tour event and sign in and take a chance, or know players at that level etc.. All troublesome.
In fact, I think I have noticed only Fyrstenberg and Matkowski, two younger Poland who successfully made that move from challengers to ATP tour in the last 18 months or so (they got a wildcard at home in an ATP event and made a bunch of points in a hurry to make the jump, if I remember correctly) .. There may be a couple of others I haven't noticed.
Jason Marshall at #129 is an example of one of the highest ranked players who has played almost exclusively challenger doubles. He is quite good in doubles (and has occasionally tried singles qualies, etc, but it doesn't seem to work out for him). I am not sure how much higher he can go without getting into some ATP tour events. Tough.
So to summarize, you better be VERY good at doubles to think of coming up through challengers and then be good enough to somehow jump from #125 to #75 in doubles rank. Then you have made it if you are good. I would advice against trying it with just doubles.