As one in time and space (2,6)
I believe the answer is:
'as one' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'in time and space' is the wordplay.
'time' becomes 't' (abbreviation).
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
Can you help me to learn more?
(Other definitions for in tandem that I've seen before include "Working together" , "Together - as on a bicycle made for two" , "'One behind the other, as on a bicycle (2,6)'" , "Together (as a pair)" , "Manned it as twosome" .)