If you are into vanity plates, here's a couple more ideas:
TANDEM
TWICER
BYKE42
TWOFER
2FER
Do not have vanity plates; however letters on our plates are DBA . . . which we interpreted as 'Double Butted *****es'
Do have a license plate frame (no gov't surcharge!):
ENJOY LIFE
RIDE A TANDEM!
And had another license plate frame for many years reading:
WE'D RATHER BE
RIDING OUR TANDEM
and before that had one that read:
TWOgether!
Pedal on TWOgether!
Rudy and Kay/zonatandem