Hey ya'll! I have an interesting problem that I really need to fix and that I can't find anything on the internet relating to.

I have two delay pedals; a Boss DD-20 and a DD-7, both of which have tap tempo inputs. I'm currently using an external tap tempo box that I built myself which has a mono 1/4" out. I'm splitting the output using a Radioshack Y-splitter, and running separate mono cables to each of the pedal tap inputs. What's interesting is what happens next... I turn everything on, tap out a tempo, and watch as the "Tempo Indicator LEDs" on the two pedals drift out of time with each other.

I know that delay is input-dependent and not time-dependent, so I know that the pedals aren't going in and out of sync. But if I stop playing at any time and listen to my delays drift off, they quickly start to bump into each other and modulate, and this causes my delays to come out of tempo with the rate I tap in to them.

I play lead guitar and often play very ambient lead lines that require my delay to be precisely on time with the song's tempo, so this is a problem which DEFINITELY needs to be fixed. Any suggestions?

(Haven't tried using a Boss FS-5u yet, and as such the company has refused to help me out so far. But the way I figure it, a pedal can't or shouldn't be able to tell the difference between a 'Boss tap signal' or a 'handmade pedal tap signal.' Right?)
Tobacco Sunburst Fender Strat
(Lollar Dirty Blonde pickups)
Fulltone FatBoost
MI Audio BoostnBuff
Fulltone OCD
This1smyne "Pearl"
Wilson 10Spot II
Red Witch Phaser
Fulltone ChoralFlange
Tech21 RVB
'59 Bassman Reissue
Are you running one of the delays into the other? That could be causing the problem. Its very unlikely to be the footswitch causing the problem, it could possibly be the delay pedals themselves, if the tap tempo input section isnt particularly sensitive on one of the pedals, or simply because they are different pedals, it could be very slightly out.
I wonder if the tap tempo function sends out a little current to check for connection (the 'tap'), so by using a Y-cable you'd be crossing the lines for those two pedals. I'd make a stereo tap-tempo switch so the two cables are isolated at every point along the way.