Think they allow you to go up to REN11 in total, REN3 from the line side of it and REN8 from the outgoing side.
I don't see why you can't daisychain 2 units together as long as you don't exceed the REN numbers above, all the unit does is when the phone line rings is pulse a relay and uses mains electricity to create the downstream ring voltages.
Noting the above, in a power cut the phones downstream from the unit might not ring.
Yes you can connect more than one as long as you don't exceed the REN for the line side as Spark 123 said.
You would need some pretty ancient equipment if you need multiple REN boosters. A typical modern cordless phone has a REN of 0.2 but may be marked as REN 1. REN refers to the amount of power it takes to make the phone ring, old bell phones took a lot of current to energize the coils to make it ring. Modern phones with an electronic ringer use milliamps of current to ring if line powered and if they are mains powered devices such as cordless phones or answering machines then they use their own power supply to ring the phone and the ring signal is nothing more than a signal to the phone using almost none of the line power.
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