never mind, it was so easy but I couldn't see it. This is how you do it:
you restart the router in rxboot, and issue a copy tftp flash command. the router treats the 2 flash simms as being one 8 mb simm, so you confirm the erasing of whatever you had on the 2 simms, and you're good to go

so now, I have this on my 25$ router
Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-IS56I-L), Version 12.0(24), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2002 by cisco Systems, Inc.
Compiled Mon 30-Sep-02 21:39 by srani
Image text-base: 0x03042828, data-base: 0x00001000
ROM: System Bootstrap, Version 4.14(9.1), SOFTWARE
Router uptime is 9 minutes
System restarted by reload
System image file is "flash:c2500-is56i-l.120-24.bin"
cisco 2500 (68030) processor (revision D) with 4096K/2048K bytes of memory.
Processor board ID 01707381, with hardware revision 00000000
Bridging software.
X.25 software, Version 3.0.0.
1 Ethernet/IEEE 802.3 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
8192K bytes of processor board System flash (Read ONLY)
Configuration register is 0x2102
fresh CCNA