I don't know how well the single cells of the more modern Symmars perform (-S, Apo, Apo-L). Supposedly the older plain Symmars, which are often labelled "Convertible" have single cells that are better corrected for coma.
Originally Posted by Soeren
A 150 and a 210 Symmar will give you a roughly 180 mm lens if you use one element from each lens. The cells are not fully symmetric (despite the name) so there are in theory four possible combinations which will all have slightly different focal lengths and aberrations. However, because the No. 1 shutter has different front and back threads there are only two combinations which are practical, so there's not so much to test.
The rear cells of the 150 and 210 used alone then give you 265 and 370 mm. The imaging is acceptable, especially if you are using LF for creamy tones rather than edge-to-edge biting sharpness.
Not a bad spread, but there's no way of bodging the equivalent of the older sets' 750 mm cell.