The donor has a choice of when the LPA should take effect.
The donor can choose invoking it automatically on registration. Alternatively, they can let it take effect after they have lost mental capacity.
I always advise the first option, because it is safer.
You should register your LPA immediately, because it guarantees the protections of the document.

Now!
An unregistered LPA is as useful as a bucket bailing out the Titanic.
An unregistered LPA is nowt but expensive paper.
Your unregistered paperwork is useless.
Send your completed documents to the Office of the Public Guardian, OPG with the applicable registration fee.
The OPG’s address is:
Office of the Public Guardian
PO Box 16185
BIRMINGHAM
B2 2WH
You should register your lasting power attorney today.

An LPA does not expire as such.
Once the deed has been granted, the document is valid till one of three things occurs.
First, donor (the person granting the power) revokes the deed.
Secondly, the Office of the Public Guardian revokes the deed or restricts the powers.
Finally, the document becomes void when the donor dies – you might need the LPA paper work when applying for a grant of probate.

Normal people recognise that illness, sickness or mere old age could leave you needing others to make decisions or act on your behalf.
With a lasting power of attorney, you can grant powers to specific people to act on your behalf. Here’s a case of someone who wish they had lasting power of attorney, but, they didn’t.
A lasting power of attorney must be set up now, while you have the mental capacity

The parties to a marriage are treated as tow individual, so each of them would get a separate document.
No, you can’t get a joint LPA.