Mobile XT -- Multiple map sets on a 6GB card

Hello! I purchased a 6 GB card for my AT&T Tilt as I wanted to keep
both CNNANT 2K8 and CNEurNT v9 readily at hand (as well as music,
etc.) I travel to Europe regularly and didn't want to have to juggle
data on a 2 GB card. Anyway, I installed Mobile XT on the card
successfully and I'm using the latest versions of MapInstall and
MapSource on my laptop running Vista. I have the map files unlocked
and can display them on my laptop. I have keys generated for both map
sets and pasted into both the .UNL files in the Garmin folder on the
card. It appears as one continuous 75-character key in each file.
However...when I attempt to install both map sets to the card,
although the install appears to go normally, I can display only one or
the other map set. I can see through Windows Explorer that the
available space on the card is down to what it would be with both map
sets installed. I go to 'Manage My Data' in XT and all I can see is
one of the map sets -- Europe or North America. I have tried every
trick I can think of to get both sets to install properly but there's
no difference. I do notice that when both have been installed, Mobile
XT takes an inordinately long time to initialize on my Tilt. Any
ideas? Thanks for your help!