When using FF miles I often need to use two airlines to get to where I want to go. In 2006 I used DL miles to fly CO metal FLL > EWR > ARN > TXL > EWR > FLL. However my desired destination was not ARN but rather CPH. I could only get a FF seat to ARN and a return seat from TXL. So I arrived at ARN, claimed my bag, walked over to SAS and checked-in for my ARN > CPH paid flight. When planning the trip I gave myself a 4 hour buffer at ARN. Thank goodness. I had weather issues in FLL on the outbound and and missed my connection for EWR > ARN. CO put me on a CO flight to AMS (A1 service) connecting to a KLM flight from CPH to ARN (worst FC EVER!). The four-hour buffer at ARN became only a two-hour buffer. After a few days in CPH I flew SAS from CPH to TXL.