This is good and bad, but either way its an experience.
I hope that this could turn into a really good discussion thread...how to turn down work and still win.
There are two obvious outcomes...but so many considerations
1) "Yes, I'd love to do it... I'll ditch the club that I'm already doing on Tuesday nights..stuff 'em"
2) "Nah thanks...I'm already working Tuesday nights ian't I"
Now, OK, I've made both the aboive examples very EXTREME to get the point across ,you wouldnt really say either of those Kev!
Thinking a little deeper, and trying to see things from all angles, there are really 4 things to consider.
1) The manager of the club which you're thinking of "ditching" on Tuesdays
2) The manager of the club which you're thinking of "defecting to" on Tuesdays
3) You !!!
4) Your DJ Career
Is your Tuesday night gig in the club that you do on other nights? If so, would they see your possible move as "A defection" or "helping the competition", "helping the enemy" etc...how would they feel? Would they keep you on for those other nights, or do they want one DJ doing all those same nights?
If you were to tell the new manager/promoter, that you'll happily defect to his club on Tuesdays, would he be pleased (of course he would) that you've accepted his offer, but feel that you're showing a lack of loyalty to your current employers, therefore a trait which you might show him/his club in the future. If you choose to turn him down, point out that you're doing so because you dont want to leave your current club "in the lurch" on Tuesdays. He'll respect you in the morning for it.
Ny no means least...You and Your Career. Is the other club better for you and your career in any way? Closer to home, more prestigeous venue with a "higher" circle of "big name" DJ's to rub shoulders (and exchange cards) with?
Also, if you choose to stay put, could you swing some more money from your current Tuesday nighter....eg: along the lines of "This guy (show card) wanted to steal me from you on Tuesdays to go an work in your rivals club, for more money. " You might get offered more money for staying put, or they might offer you better incentives to stay put eg: Free drinks (including soft ones), free entry to linked/associate clubs, free choice of stripper to take home at weekends

etc...
If you can win in all four aspects (theres no doubt a lot more aspects too) then so much the better. Keeping everyone happy is an aquired art, and not just while the mixer is on.