Many transmitters could have mixing as an option anyway but if not then: if you arrange for the mixing to be done at the receiver end and before the esc connection then you need not worry whether you are using a brushed esc or a brushless esc.

So you can have a 2 channel RC set with throttle and rudder control including twin differentially controlled props.

I have a couple of units that are very small where the i/p wires from the mixer throttle and the i/p from the rudder are first taken to the Rx. The o/p from the mixer then has the 2 sockets for the esc's to be plugged in.

Sometimes called a V-tail mixer for aircraft. There will often be a single pin (i.e. no normal servo connection) and this goes into the signal position in the receiver (rudder socket) into the signal or innermost connection of the black, red, yellow or other colour.

The units are not easy to find as using a search engine the preferred 'Mixer' word is looking for mixing with audio speaker amplifiers as a first choice.

The first link is for a UK made unit.

The second one is for a cheaper but similar unit. Although this is called a brushless mixer it is clear from the description that it is to DRIVE a brushless esc. That is that the mixer does not have an internal esc. The receiver does not know what kind of esc is connected. Most aircraft these days have brushless motors. The i/p wire is a special 4 pin one but you can see the other end is a standard servo lead and the single signal wire referred to.

The photography of these units it is sometimes difficult to realize that there are 2 servo o/ps one on top of the other, usually a wiring diagram reveals the second o/p maybe in a colour change. In the second link the o/p pins to the esc are side by side, the polarity is printed on the board.

You should be aware that although you have 2 esc's there is only one throttle control on the transmitter. Usually up/down on the left. The rudder control on the Tx. will decide on direction and without further wiring there will not be any connection for a rudder.

If you wish to have a mixed / differential speed control and a rudder working then a little extra (simple) wiring is needed.

The single signal wire which plugs into the Rx. will block the use of a rudder servo plug. If you want both, then use a 'Y' lead with the single end male to plug into the Rx rudder socket.
You then have 2 ends to plug into for the rudder control. Plug a servo into one female end and the single wire into the the signal socket of the other one.

This will be a not red and a not black wire but the signal wire which could be several of the lighter colours.

You may have to change the esc's over if the opposite working is connected.

All that are needed are available, servo wires as described, so no soldering etc needed.

Hope this helps.
Roy

https://www.cornwallmodelboats.co.uk/acatalog/mtroniks_wtail.html

https://www.fruugo.co.uk/brushless-dual-channel-mix-controller-two-way-speed-dif

I have seen, many times, where one of us is trying to increase the turning ability of their boat/s. Action Electronics made? or still makes a mixer that performs the function of increasing the outboard throttle when to appropriate rudder direction is called for, but I believe that works on only brushed motors.
Is there a way to do it for brushless motors? Yes.
I am learning this as a novice RC airplane pilot.
This system applies to brushless motors where two ESC's are used. It requires a "computer radio"( aren't they all now?)
The following link explains how to easily program the mixes and program the switches.
This system works very well on my Spektrum, I can attest to that, but is applicable any to descent radio.