You can, however, either make an adaptor to connect a normal IDE cable to the NA or solder the IDE cable to the NA...
But why would you want 2 HDDs? No software supports it, not even software from Sony. A lot of work will be involved to add hdd1: support.
The TOOL might support multiple disks on each IDE channel it has, but it runs with PS2LINUX and not only with the regular PS2 IRX modules that games come with.
I do imagine that Sony did intend the SCPH-10000 line of consoles to support multiple disk units at some point in time, since the SCPH-20400 HDD unit has a unit number selector at the bottom... but that was never used at all.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit