I know you can do this in some countries, but I haven't seen anyone post about doing this successfully for Vietnam...although I did search and find people asking about it. It does sound like the VOA line has gotten a lot shorter due to the E-Visa so if you need multiple entries within a 30 days period, you may just want to apply for that VOA to be safe.

If you do decide to try using 2 eVisas...let us know if it works...