I have done the same too. If you have multiple people on the same card you get a screen where you select which person you want to check in. On one occasion I checked in 18 people at Hamburg via the SSCI machine all on the same card (multiple PNR's with 3 or 4 people on each). On the outbound from LHR some people did SSCI and some did manual check-in without the credit card holder (me) present and the SSCI's were done in batches as people arrived (not all together).