I don't think you'll find a stock cable to do that simply because the Bluetooth speakers will have a stereo input and you want both tip and ring on one side to carry the left signal, and tip and ring on the other side to carry the right signal. If you use an "insert cable" (which are mostly available for 6.3mm though), you end up with two mono plugs which effectively have the ring shorted to ground, meaning that the right input channel on either bluetooth speaker will be dead, resulting in only half the speaker doing actual work.
So your best bet is likely to wire/solder your own splitter that has one TRS plug for the input source and two TRS plugs for output, and then you wire T (tip) from the input plug to TR (both tip and ring) on the left output plug, R (ring) from the input plug to TR (both tip and ring) on the right output plug and, since you'll be using shielded wire for that, run the shield from the input plug to both output plugs' shield.
The laziest way to solder this is likely getting the cheapest earphones you can find (dollar shop?) and cutting off the earphones, replacing them with 3.5mm TRS plugs that have T and R wired to the center wire and S to the shield.