Mike Lyle writes
I've got a pelargonium on the bathroom windowsill, and have encouraged
a couple of horizontal stems to grow across. I'd hoped these, now a bit
over two feet long, might send out flowers at intervals, like some
roses, but after two years they're still obstinately flowering only at
the tips. There are some little shoots, but these never develop
full-sized leaves or flower-buds. Is there anything I can do to
encourage flowering, or will a pelargonium never adopt this habit?

IME, they only flower at the tips. You'd be better to start new cuttings
in separate pots. Or grow something else.
--
Kay