In agreement with Earl here....
I run Barudan also, its tough to justify buying a 2 head when 2 singles are cheaper, and no doubt more versatile. The reason why according to my Barudan rep, (and it does make sense I suppose) is, because there is A LOT more material going into making a 2 head, because they are a beam style machine, (the head is mounted on a horizontal beam over the sew field. VS. an arm style where the head is mounted on an arm that hangs over the sew fiels. It takes a lot more metal to make that huge frame of a 2 head, then just the arms of a single.
However, if you look at the beam style single heads. (The model C01) 2 of those are not cheaper than a 2 head. Its got to do with the frame style and what it takes to manufacture.
Now as far as the 2 vs. 4. It depends on your business, what do you do more of? Onesies & twosies that people come in off the street for? Or does your average order range in the 12+ pcs. range?
I say this because, if you are doing a lot of onesies and twosies, then running 2 pcs, on a 4 head is rather a waste of time/money. You figure, to make the max amount of money, you need to have as many heads running at once as you can. So if you are constantly doing a lot of runs under 12, then in a sense a 4 head is a waste, because then 1/3rd of the time, you will have one or more heads shut down. Where as a 2 head (from a percentrage standpoint) will have more total uptime.
Have you entertained keeping your single, and getting a 2 head? Perhaps a 2 and 2 singles? Machine companies love to deal in bulk, wheel and deal with them. You generally can't get major price breaks on singles, cause most companies have no problems selling them, but try anyway. See if they will cut you a deal on a 2-head and 2 singles, or 2 2-heads?
According to my Barudan rep, 2-heads are the slowest selling machine. He claims they sell more 12 and 15 heads than 2 heads.
Perhaps not the most cost effective but, I love my 2 head machines, they are great for customers that come in off the street and just need a few jackets, or a few caps done for an event or something.
Just my 2 cents on the 2 heads