3.3V for both works, normally you only have the options of 3.3V and 3.5V on single voltage boards.

I do not think there are any issues with running at 3.3V except that you need better cooling, I have even tried 3.5V when overclocking a Pentium MMX with very good cooling.

The P5TX-Bpro does 2.5 2.8 2.9 3.2 3.52, but only one of them. Probably you can get away with any of those, as the old Pentiums were pretty tolerant chips. But my question is what you were /meant/ to do when faced with a single-option board like this one.

You are meant to buy an overdrive CPU for double the money or buy a new board.

There is also a PowerLeap adapter.

Supported CPUs This mainboard can use CPUs from Intel, Cyrix, IBM and AMD. The board’s switching CPU power design and jumper con […]
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Supported CPUs
This mainboard can use CPUs from Intel, Cyrix, IBM and
AMD. The board’s switching CPU power design and jumper con-
figuration options allow the use of all Pentium class processors
from all three vendors, including those with MMX features. The
correct jumper configuration automatically sets the required
power configuration for the CPU.
Processor speeds from 90 to 233MHz are supported as well as
single and split voltage CPUs.
Intel CPUs Supported:
Pentium P54C, P54CTB, P54CT, P55C
Cyrix & IBM CPUs Supported:
6X86, 6X86L, 6X86MX
AMD CPUs Supported:
K5, K6

Ohhh... now we're getting somewhere. With keyword 'split' instead of 'mmx', I find

CPU voltage may either be the same internally and externally or it may be split, depending on the CPU design. Some proces- sors […]
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CPU voltage may either be the same internally and externally
or it may be split, depending on the CPU design. Some proces-
sors use one voltage for the ‘core’ (Vcore) and another for input/
output (Vio).

Intel's datasheet has no reference to a Vio, refering only to Vcc2 and Vcc3. CPU-World calls Vcc3 the I/0 Voltage, so that's probably how things work, though it'd be nice to have an Intel reference for it.
http://www.cpu-world.com/sspec/SL/SL26Q.html

In which case it seems reasonable that the manual's reference to JP3 as "CPU Core Voltage" means Vcc2 for MMX chips.

That is, if you're willing to overlook how well Taiwanese motherboard manuals usually apply English. That's a bit of a leap, I admit.

There we go, it /was/ in the datasheet:

The V CC2 pins supply power to the Pentium processor with MMX technology core, while the V CC3 pins supply power to the processor I/O pins.

But anyway, my guess is that since all plain Pentiums are 3.3 and all MMX are split 3.3 and 2.8, and since the P5TX-Bpro has that remarkably wide voltage range for its single setting, that it must be intended to follow the lower VCC2 2.8 when using MMX chips.

There's a "VCC2 detect" pin on MMX. It might use that to know when it should supply two voltages. Or perhaps they've done simpler and the 3.3 side of the split remains at 3.3 regardless of JP3 and that still works okay for plain Pentiums.

I can confirm that a pentium 200 MMX with 2.8V core voltage and 3.3V external voltage runs absolutely fine on a 3.3V single voltage socket 7 motherboard (packard bell D130, motherboard PB680, BIOS rev 1.00.05.DNOR in my case). I ran such a configuration for at least 5 years using a standard socket 7 fan; even with just a heatsink the chip would not get unbearably warm. With a TNT2 M64 32Mb RAM, Quake II would fly on this configuration (with 64Mb RAM), was lots of fun!