I take it this is a Millermatic 250 and not a DialArc 250?
Don't know about dual solenoid valves, generally questions like these can be answered by reading the manual, obtainable from the Miller web site.
The Millermatic 250 is the last of the analog-controlled 200-amp MIG welders in the big rollaround case. It has the same basic specs as the 251 and 252, but it isn't digital and of course doesn't have things like pulsers. More to the point, it does not have built-in spoolgun support. You have to add the spoolgun module, part no. 043084, currently selling for $227.20 on miller4less.com. Then a 30A spoolgun will hook right up. Don't know if your 212 spoolgun will fit. Yes, you can then use both the spoolgun and the regular gun at the same time, assuming your gas connections are made to both. The regular gun's gas connection is at the back of the welder, while the spoolgun's gas connection goes directly to the regulator on your cylinder.
I love my MM250. I got mine for $500 with a cylinder of gas, added the spoolgun module and a 30A spoolgun, and have made about $100k with it since.
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