Forget what the law may state, HMRC do actually take a pragmatic approach.
If the 2 people are getting roughly 5k p.a and are not in receipt of benefits, tax credits etc, then put it through as wages, and claim it legitimately as a business expense.
If you register under a PAYE scheme you are automatically open to lowly trained PAYE compliance personnel who want to hit you for fines to justify their existence.
It is all down to whether or not you are aufait with the two peoples circumstances. One of my clients employs his wife and his student son (effectively 10k a year tax write off), with no PAYE registration nor anything else. All he has to do is justify that they do work and that 5k each is reasonable. He knows neither claims via DHSS nor tax credits, and as he is a 40% tax payer this little bookkeeping exercise saves him in excess of 4k per annum.
Officiallly he should register as an employer blah balah blah, but in his case the first point of enquiry would be an Inspector of Taxes and not a muppet sent out to generate revenue for the compliance team. And guess what, under the framework of tx, the inspector has discretion that the muppets don't have.
If you have concerns that your "employees" have other sources of income, then yes, apply the PAYE procedures, and DO NOT RELY on invoicing from them. If you are their only client, and they can demonstrate you should reasonably have know this, then you are liable for some "spank the boyo" money.
There is another way around it btw, but wont post in an open forum, as the relationship you have with these people may govern its applicability.