That´s a working setup. You have a total of 512K L2 Cache and the TAG-RAM soldered on the mainboard together with the one on the COAST module provides the necessary 11-bit TAG-bus width for enhanced cacheing mode.
The cachable area will be 512MB in this case.

Edit: You need to use a COAST (3.0) module with two TAG chips in this case, or, if your COAST module is a version with just one TAG-RAM you MUST populate the second DIL socket on the mainboard or your L2 cacheable area will be restricted to 64MB! (see my posts below)

If your board had 512K (= 2 x 256K chips) SRAM L2 cache onboard the TAG-RAM DIL socket would need to be populated also with a second TAG chip for the same result. In this case you don´t need a COAST module and that´s why manufacturers didn´t solder in the COAST-socket anymore in this case.

(This is because the HX Northbridge is limited to 512K L2 cache. Other chipsets like VIA can deal with larger L2 caches. Some VIA boards have 512K L2 cache onboard plus a COAST socket for another 512K. In this case there are also 2 TAG-RAMS of 32Kx8 present.)

Thanks! 😃👍
So the DIL socket is probably there in case the user wants to use a COAST module without TAG RAM chip? I think I saw those before.

The additional I/O lines TIO8-TIO10 are not present at the early versions of the COAST slot - there´s only TIO0-TIO7 (at pins 2,3,4,5,82,83,84,85)
If you insert a COAST module with only one TAG-Chip into these slots it will be working together ("parallel") with the onboard TAG-Chip WITHOUT enlarging the TAG-bus width to 11 bit.

download/file.php?id=82702&mode=view
In cases like this the onboard TAG-RAM besides the COAST-Slot will / must be connected to TIO0 - TIO7. When there´s 512K onboard cache present, or a COAST module with additional 256K and just one TAG RAM on the stick, the TAG-RAM in the DIL socket will take care of TIO8-TIO10.

The later COAST versios (3.0 and above) have been modified and connectors for TIO8, TIO9 and TIO10 have been provided at three contacts that have been "NC" before:

If there´s only one TAG-Chip/socket onboard and you add a COAST module, the module MUST be a version with two TAG-Chips - one on the front besides the L2 SRAMs and one on the backside. The chip on the backside is wired to COAST -socket pins 6, 86 and 88 for TAG-bus I/O lines TIO 08 - TIO 10. Then and only then you get the broader 11 bit TAG-bus width and accomplish the full L2 cacheable area of 512MB of RAM.

But look at this example of an ASUS P/I-P55T2P4 (a late revision 3.xy):

The COAST slot is not present, so I assume the onboard L2 cache is 512K. Then there´s one TAG-RAM onboard that allows a maximum cachable area of only 64MB
To expand the cacheable area to 512MB you had to populate the DIL socket with a second TAG RAM.
I don´t think a TAG-RAM (16Kx8) was THAT expensive back then. Either it has been removed by a former owner or ASUS didn´t populate it to save some money.

Today I did some further research and some tests with an Elitegroup "P5HX-B" (Rev. 1.1 with split voltage):

There´s 256K L2 cache onboard plus a 32Kx8 TAG-RAM and there´s a COAST slot.
I updated to BIOS 1.6h (the latest one that I know), jumpered everything carefully and inserted 512MB FPM RAM and a regular 256K COAST stick.

At POST a total L2 cache of 512K is detected correctly. Then I turned "cacheable area 512MB" on in BIOS setup and started the system.
No surprise that I was stuck at a cacheable area of 64 MB no matter what I tried, because there´s only one TAG-chip onboard and a second on the COAST module - both of them wired to TIO 0 - TIO 07 as we know now.
With an ASUS COAST 3.0 module with 2 TAG-Chips everything´s working perfectly.

Alright! Thanks for further researching and updating!
Gotta get me some TAG RAM then!😄