I'm working with an area spanning two UTM zones. The magazine I will submit my work to needs the projection in UTM.
Is it possible to combine two neighbouring UTM zones to present a map/data that spans them?
If the extent of your map falls within 2 UTM zones, and you are required to use a UTM projection, you could either:
1) Choose the defined UTM zone that more of your map extent falls within. For example, if you are between zones 34N and 35N and more of your map coverage is on the 34N side, create your map in the standard EPSG 32634 and specify that this was used.
2) Locate the center of your map coverage extent, and set this longitude value as the central meridian for a transverse mercator (UTM) projection. Provide the projection parameters to your publication (as long as nonstandard projection would be acceptable).
You really should push back and mention that this is not a good idea. Pick a custom projection or a standard coordinate system that will better fit your project area.
As you move from one UTM zone to the other the coordinates are going to cause a problem, If your in Zone 17 and 18, and you chose 17 as the map projection, all the points in zone 18 will be in meters but based off of zone 17's origin. Now later someone wants to plot those points and they know they are really in zone 18, those coordinates won't make sense to them.
Often the client doesn't really understand what they are asking for, so offer a better suggestion and explain why. I used to get the request to make a county road map with all the roads and labels and can you put it on 8.5x11 paper so it's easy to carry.
If it's a once and done graphic you might get away with it but if your going to use the data later it's better to fix the problem now.