I believe MekHQ cannot, at the moment, track player units spread across more than one system.
There is a mechanic included for subcontracts occurring on the same planet, and you can take as many of those as you think you can fulfil, but as you are only on one planet at any given time, you can't fulfil more than 1 full contract at a time (unless you're super lucky and it ended up on the same planet?).