When hooking up the hoses, there will be a transfer of air (and likely moisture) that resides in the hoses which will contaminate your supply. Also, it will be difficult "forcing" the refrigerant into a system that does not have vacuum pulled on it. Finally, you would need hoses hooked up to both high AND low sides on both cars in order to overcome the expansion valve and ensure you get all the refrigerant out of there, and I don't know of any manifold gauges or hoses that have 2 high and 2 low connections.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280