I can't comment on that specific combination, but I do know there's been cases in the past where one airline (where you could check partner availability on the airline site) in an alliance showed more partner activity (at a third partner) than another airline in the same alliance "saw" (whenever you'd call in to actually book). I also know of cases where one airline in an alliance can show nonstops of another ailrine in the same alliance online, but online returns "no result" if you search for a route where the other airline can only do a one-stop-or-more connection.

So before thinking this is a general solution, you have to do some research to see whether the airline B that you're using check alliance X seats online accurately reflects the real availability of the airline A whose miles you plan to book with.

You thus often have to use multiple websites from within the same alliance to see all possible routings online.

Another issue is that an ailrine often makes award avilability better for its own members than to partners. So, for example, even if airline A may show accurate award aviailbility of airline B when using airline C's miles, it may not show accurate award avilability of itself when using ailrine C's miles.

If that were the case here (which I don't know one way or the other), trhen Areoplan might be good if you were trying to search for United availability of Lufthansa flights, say, but might not be good if you were trying to search for United availability of Aeroplan flights.