Up until very recently I was using an Icom IC705 with a 30ft RW for transmit and a MFJ1886TR Loop for receive. The Loop is supplied with its own switching box with an extra SO239 for the transmit antenna, which I had connected to the SEND/ALC line of the 705. When in transmit the loop antenna is grounded and the BiasT is switched off, and the RF diverted to the TX antenna port..

I now have a Xiegu XPA125B amplifier which is switched from the same SEND/ALC line of the IC-705. Is there a simple solution whereby I can split the SEND/ALC line between both the amplifier and the control box of the loop? Could I simply make a spitter box with a one in two out configuration?

Depends on polarity of switching and the total two devices switched "load".
If the polarity of switching is the same for both items, and total switching load doesn't exceed what the 705 is capable of switching, you could switch both with just a couple of series correct polarity diodes, (just for switching paths isolation).
If they differ in polarity of switching, or the total switched load exceeds the 705's switching capability, a suitable switched external multi pole fast action really circuit from the 705's switching output would be best, and act as isolation of loads buffer between the two separate switched
loads, the 705 switches the "buffer" low current multipole relay, and separate relay contacts within it switch the two desired outboard items.
Personally I have always opted for an external switched multipole relay when switching multiple external devices from a single transceiver TX PTT output socket, as it then doesn't matter what polarity the switching circuit is of the different external devices, or what switched current they draw, or indeed what "open circuit" unswitched voltages they exhibit at the TX switching point.

I should say the diodes switch in parallel, they are common connected at one "switching end" to the 705's switching point, be that to v+ or ground, polarity conscious of course, the switching side of them presented to the two different devices is of course in series with the two devices to be switched, PTT line be that ground or v+.
This only works if both external devices are either bith positive switched, or both ground switched.
If they differ, ie one need a grounded PPT line, the other needs a v+ signal, then an external relay really is your answer.

Hi Dave, and thanks for the replies, apologies for my tardy response.

The amplifier, the switching box, and the 705, use a simple switching, essentially to ground. I have come up with a simple but elegant solution, and that is to use a regular headphone splitter, one in / two out. Both the amplifier and the switching box are in perfect sync. My worry was that the antenna switching box wouldn't switch quick enough and I'd end up with RF going into the receiving loop's HI-Z amplifier. This happened a few weeks ago when I forgot to connect the PTT line to the box and the HI-Z amplifier was toasted. To MFJ's credit they sent me a free replacement via Fedex, two days from Mississippi to my front door, even though I told them it was completely my fault and was willing to pay all costs.

Ouch!
I did that once with an homebrew ptt switched masthead pre-amp on VHF!
Ideally you could do with a sequencer of sorts, just to ensure the antenna switch box switches first each and every time to the the correct TX antenna, followed by the amplifier switching.
One way you could do that is to modify the antenna switching box to provide its own grounded ptt output connection, that only grounds once it has correctly switched over to the TX antenna, connect that to the amplifiers ptt line, then switch the antenna switch box only from the 705, and let the antenna switch box itself bring in the amplifier on completion of its TX antenna switching operation.
That way you could never by virtue of a forgotten plugged in cable, or dodgy connection have the amplifier kick in when the hi Z pre-amped RX antenna was still inline and prevent another "frying".
Just an idea.