Unfortunately, I can't comment on specific brand names but, some 486 motherboards require two pairs of simms due to memory interleaving (which is kind of a forerunner to dual channel memory).
Also, the reason why your two Pentium simms don't want to work in your 486 PC is because it is most likely EDO memory. Some 486 motherboards will only work with fast page mode (FPM) memory.

I haven't been able to distinguish between the two types by just looking at them.
You either need to test them in a motherboard that only requires FPM memory or, look at the model (serial) numbers on the chips to try and identify which type it is (I recently performed such an exercise with a couple of simms I have and was fairly successful in identifying the type of memory)

If you combine 70ns and 60ns memory modules, then I would imagine that the overall speed would be that of the slowest component (70ns in this case).

I usually disable shadow RAM and the memory hole. The latter, if memory serves correct, is only required for very old ISA cards.
You can, however, enable caching of your system and video bios as well as caching of your video RAM to improve performance.

Also, if I remember correctly, some 486 boards don't like 72 pin simms with chips on both sides of the module, but that may have just been for odd sizes like 8MB, 32MB, and 128MB.

Make sure you get FPM RAM. EDO is hit or miss in a 486

some 486 chipsets claimed to support edo dram after all, just wonder which one your board has.

First you need to know if these sticks are FPM or EDO.. Otherwise it would be very complicated to determine which stick will work in your motherboard, and which dont..
Because really the older 486 boards (most) will only work with FPM memory.. EDO memory appertain more to the newer 486 boards and mostly used on Socket 7 and or Super socket 7 boards.
If you put an EDO stick in an FPM only 486 motherboard, you still would getting the black screen after power up the system.. And often it would also could give some beeps through the pc speaker to let you known the ram isnt working in your motherboard.

To tackle all of these finicky problems iam using an old innoventions memory tester.. I think that it would be expensive for some people, but i think it saves a lot time of if a module would be faulty or not and really knowing very fast the specifications of the tested module. I know it was expensive, but it was a great investment for if i would retest the modules, or when there arrive more old modules here..

I am pretty sure that digging up these forums will provide all the necessary info on a rudimentary check-up, without having to spend mega bucks on a RAM tester. Keep in mind that these are 20+ yrs old themselves, yet brutally overpriced on ebay. Of course, if one builds retro rigs on daily basis then the price *might* be justified.

When the computer can boot, you can run the FITTING version of memtest or memtest+ a few rounds to make sure it is OK. (Some versions can give false alarms on 486 or lower systems.)
From memory (no pun intended), the odd number of chips on a memory module, when one has different size and labelling indicates parity RAM. But parity RAM is SLOWER than non-parity (but can be counted on not to freeze your rig on errors)

Also, many chips' serial numbers end on something like "-60J" "-70J" etc. (rated speed). You take a look at the two digits that PRECEEDS these and if it is 0 (zero) it is FPM RAM, if it is anything else (very often 3, 5, 6) it is EDO RAM.