Can I perform 2 missions at the same time if they are in the same store?

This is technically possible :)

If you spot that there's multiple missions open for one location, you can decide to perform both missions (less travel!). Just click the pin on the map and reserve the mission, then scroll down to the 2nd mission and tick 'reserve the mission' again if you wish to complete both.

Note. In order to reserve multiple missions at the same time your level needs to be 3. Meaning if you have less than 300 XP it won't be possible to book both missions at the same time. It will be possible to book the 2nd mission after you've completed the first one - but remember it can be booked by someone else too.