Hi Laura
If you want to keep it really simple ( you say you’re a newbie) I’d just use one mic and get really close (knee to knee). I do a lot of interviewing (yes often in offices) and if you have two mics, yes your interviewee may be more comfortable but also may rabbit on quite a long time… which will add to your editing time.
I don’t know the H5 but use the H4 as standard. Test the mic and your equipment before you set out. ALWAYS use headphones, that way you can hear any pops or if the battery cuts out (not that you would turn up with dud batteries!?). Make sure you are not listening to the built in mics by tapping both and push the mic button on the left on or off (I can never remember which) to make sure you have the correct setting. This is probably all grandmother and eggs, but worth saying I suppose in case anyone else is reading. Then if you don’t get really close you’ll pick up the vast office and less of your interviewee.
Hope this helps and good luck!
Nicky H