Sounds like a classic interrupt conflict

Re: Dual PCMCIA NIC system (Linksys plus NDC ) time outs and lost interrupts (Dale moberg)
Date: 2000, Jan 30
From: David Hinds <dhinds@pcmcia.sourceforge.org>

You have an interrupt conflict with some other device on irq 5. The PCMCIA-HOWTO actually even describes this exact scenario (where you've got two cards that each work separately, but together, only the first one works). -- Dave

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1. OK, I guess next time I will just ignore proc/interruptsI by Dale moberg, 2000, Jan 30

to: "Sounds like a classic interrupt conflict"