Nope. Fantasy leagues are self-contained entities. Assets within a league can only be traded for other assets within the league.
It'd be just as non-kosher for two guys to join a pair of leagues, and for Guy#1 to trade his top three picks in league A for Guy#2's top three picks in league B. It's not fair to the rest of the teams in either league, and it'll destroy any semblance of competitive balance.
Now, if it's a sufficiently friendly league and the pieces involved are sufficiently worthless, you might carve out an exception for it. Maybe allow guys to trade their 19th round pick for a 20th round pick and a beer at the live draft. Maybe allow teams to make real-life bets using crappy players at the end of their rosters as stakes. Maybe allow someone to swap waiver priority as a "thank you" to a buddy who just did him a huge favor. You might decide that the amount of fun it adds to the league is enough to outweigh the extremely minor ripples it causes in the competitive fabric. I've had a few leagues like that, where basically the whole point was to get together with your buddies once a year, get wasted, and have a fun afternoon. I've seen a few trades go through involving outside stakes, and never had a problem with it. The key, again, was that all pieces involved were so miniscule that any impact it had on competitive balance was basically nonexistent.