In article 5mj1j.18273$701.5782@trndny08, "Smarty" wrote:
I've just had an unexpected failure of my 6 year old sump pump, resulting in
a fair amount of rain water in my basement.

In considering replacements, the thought occurs to me that a 2nd redundant
pump placed above the replacement primary pump would hopefully prevent this
situation from occurring again.

That would protect you from a pump failure, yes -- but what about a power
failure? For your second pump, consider something with a backup battery, such
as Basement Watchdog or Ace in the Hole.

The sump is 18 inches in diameter and 28 inches deep, enough space (I think)
to mount two pumps, one above the other.

Or side by side, which is almost certainly easier.

I am a bit uncertain as to how, if at all, I should use one-way valves in
the discharge plumbing to prevent backflow.

You should always use one-way valves.

The discharge line is 1-1/4inch NPT. I am wondering if just using a "tee" to
join the two discharge lines would be sufficient?

I'd use a wye instead of a tee (less flow resistance for the arm on the side).

Any thoughts would be most appreciated and thank you in advance.

Having two backflow preventers, one per pump, installed on the pump side of
the wye, will make installation (and later service) much easier, as opposed to
a single backflow preventer on the discharge side of the wye -- provided you
use the type with rubber couplings and hose clamps, not the ones with threaded
fittings.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.