When I run my 35a Vector 1092A (14.6v) with my 40a (14.8v) VEC1093DBD I have to start the 35a first and get it running, then turn on the VEC and it ramps up so now I get 75a. If I start the VEC first, and then start the 35a, it won't run, says "full" or gives a fault number.
Some have found that you have to start the VEC first when charging with a converter at the same time, even though the converter is a touch lower in voltage than the VEC. Each situation seems to require a bit of trial and error till you find the trick for that combo.
So if one won't run, start it first and then start the other one and you could be in business.
AFAIK, solar and converter don't care about all that. Maybe it is a portable charger thing.