Hi,
You can use a drum gate to run a sequencer with an analogue clock input. You can use a drum instrument track to advance the sequencer. If your sequencer advances one step per pulse, send it one gate every two steps for 1/2 speed. You can't double time your external sequencer like this unless it runs on 2ppq (I'm looking at you, Volca and TE owners, you actually have a 'hidden feature', not a 'bug').
MarkD