An Instrument Rating is valid for 12 Months and will be revalidated by Proficiency Check. A Multi-Engine Instrument Rating will normally be be revalidated by proficiency check at the same time as revalidation of the aeroplane rating,
but may be renewed separately.
If your MEP class rating is valid, you can renew your IR in simulator (indeed it's cheaper)
If you don't have anything valid (class/type rating), for reducing cost it will be better to renew both in one flight (IR-ME/MEP).
In EASA land, you cannot have an stand-alone IR(A) without a class/type rating valid. Before it was possible in JAA land.
EASA instrument rating - IR(A) revalidation | Flight examiner ? re-validation, flight school, flight traiing
EASA instrument rating renewal IR(A) | Flight examiner ? re-validation, flight school, flight traiing