The G-K is an old combo cab (amp head missing) and the tweeter horn is laying on top. There might not be any crossover (LPF) for the horn in the cab. Never seen two tweeters in a Hartke 15" cab, could be bogged together. I'd probably do it for $40 (20 a cab) especially since he announced he's sick of looking at them.
IF THEY WORK.