Arrangement of two things in time and space (6)

I believe the answer is:
tandem
'arrangement of two things in' is the definition.
Both the definition and answer are adverbs. Maybe they are linked in a way I don't understand?
'time and space' is the wordplay.
'time' becomes 't' (abbreviation).
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
't'+'and'+'em'='TANDEM'
Can you help me to learn more?
(Other definitions for tandem that I've seen before include "it's not normally what one rides" , "Bicycle for two riders" , "One behind another, as on bicycle" , "Bicycle for two (plus)" , "Bike with two saddles" .)
