Hi there,
It sounds kind of wild, but I don't know why it won't work ok - I'm not sure that you want to tap the high side, though, especially with the donor car running. At the very least, transfer all you can get before starting the donor car to avoid slugging the compressor on the other car with liquid freon. Let us know how it worked!

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C