Ed wrote:

I don't know what KISS stuff is

Well-established acronym for 'keep it simple, stupid'

Kisses and hugs (in the purest of senses, of course) to you mate for
your very helpful reply.

Yup. Go to your local fiendly hardware shop / DIY store and get two
screw-up nipples and a hole-saw of a suitable size.

If necessary, get a length of good quality hosepipe and two clips (I
don't like Jubilee clips and would advise against them) which contract
all round. Hose-clips from a motor factor are favourite IME.

Cut a hole near the bottom of each butt, and fit the gubbins, raising
the lower one to the level of the higher. That way they'll both overflow
at the same time. Make the lower one slightly lower than the higher, and
it will overflow first, ensuring your shed/greenhouse/w.h.y? doesn't
have wet feet.

--
Rusty