If Cisco 3845 has following 2 Flash Memory,
-31360K bytes of ATA System CompactFlash (Read/Write) <------- 32MB Internal CompactFlash Memory installed
-31360K bytes of ATA Slot0 CompactFlash (Read/Write) <--------- 32MB External CompactFlash Memory installed
Can I install IOS which require Flash Memory 64MB on 32MB Flash ×2 ?
or Should I upgrade to Compact Flash 64MB from either 32MB ?
Are you sure this is not a 3700 router because I don't recall the 3800 having an internal and external CF.
I'd recommend you upgrade to a bigger CF. The 3700 and 3800 will support CF up to 2Gb. Format the CF first before loading anything into it.