The Oxford English Dictionary says
Definition of tandem in English:
tandem
Line breaks: tan¦dem
bicycle with seats and pedals for two
riders, one behind the other
Having two things
arranged one
in front of the other:
As they offset the seats so Guy was slightly (30cm ish) behind Jason , its down to debate as to "one behind each other" as the dictionary does not say they need to be "in Line"
If you were in a race, and you went over the line a tyres width in front of your competitor , the looser would be classed as being behind you , although they are actually almost at the side of you.