As Radu094 has already mentioned, minimizing the gate turnaround time is usually the number one goal for budget airlines. It is thus crucial to use both doors to board/deplane passengers, as that can obviously almost halve the necessary time. In my experience (about 100 flights in Europe a year, maybe half of that with EasyJet), this is the norm and boarding only via the front door happens very rarely.
I have never seen a budget airline use one of the posh dual-jetbridge gates to board an A320, so that inevitably sends some passengers to the tarmac. On some airports, the design of the gates makes it possible to use both the jetbridge and the stairs at the same time (the front half of the aircraft boards via the jetbridge, the other half is sent to take the stairs). If this is not possible for some reason, then the jetbridge simply won't be used.
In fact, at some airports (for example PRG), there's usually a gate agent standing at the junction of the stairs and the jetbridge, checking everyone's boarding passes and sending them the right way. Other airports rely on information screens or instructions on the boarding pass like "rows 1-15 via the front door, rows 16-30 via the rear door", but that's much less efficient as there are always some folks taking the front stairs even though they're seated in row 28, which .