I take it from "double-shower" you mean a shower that two people can use simultaneously, not just a "bigger-than-normal" shower?
I haven't "put in" a double shower but my present PPOR has a double-shower, which we specifically requested when our PPOR was built.
Showering together saves a lot of time. Having had a double shower, if I had to rent I would definitely consider a double shower a worthwhile feature over a home without one.
Shower base measures 1500x900, and personally I would not consider anything smaller for two people to actually shower comfortably.
Some suggestions:
1. Personally I wouldn't bother unless the bathroom/ensuite is large enough that two people can comfortably get dried (etc) in the same space, outside of the shower enclosure.
2. Give careful consideration to how your shower doors will work. Pivot doors encroaching into the shower base can be a pain in the ... My preference is for sliding doors.
3. A lot of plumbers will put in one set of hot/cold taps and the "other" shower rose simply has a stop-cock to turn on the water flow. This means both showers get the same water temp. Personally I would not do this. Spend the extra $100 and put in independent taps. Besides looking nicer, not all couples like the same water temperature.
4. Be wary about total water consumption and that your HWS can deliver it if it's one of the "instant" HWS types. Not really an issue if you have a mains pressure storage-type unit.
5. I think a double shower works best when you have 3 tiled walls and one side screened. Recommend putting taps opposite one another on the "short walls" (same walls as the shower roses), and also two soap trays (one on each wall).
6. Personal bugbear of mine. If you get a plumber to do the work, tell him the height at which you expect the shower rose outlet to be. At the time our PPOR was being constructed, the builder's "standard" height for the outlet was 5' 6" above shower base level. I have been in motels where I swear the shower is 5' above shower base level. I am 6' 4" so I notice these things. 6' above shower base will accomodate nearly everyone, if you don't have a "fixed" shower rose.