If we issue two separate tickets with different PNRs, and the connection flights is within 24 hours, do the travellers still require a transit visa?
That depends a lot on the details. Two different PNRs can be tricky. The troublesome part here is
THEY MUST STAY IN THE INTERNATIONAL TRANSIT AREA OF THE AIRPORT
With two different PNRs you need to collect your luggage, go to the check in counter of the second flight and get a boarding pass and drop of your bags, and re-enter the secure area. That means you are NOT staying in the international area. In many airports you can't get back from the baggage claim to the secure area and you often also have to pass through customs and immigration along the way. Some airports don't even HAVE an "international area" (mostly in the US and Canada).
Without checked luggage things are a bit easier. If you can check in online or at a customer service desk, you can get a boarding pass of your second flight without having to leave the transit area. However, whether you can get to your departing gate staying inside the transit area depends a lot on the layout of the specific airport and the terminals/gate.
There is also a good chance that you will be refused at check in for your first flight. Let's say you are flying from A to B to C. At airport A the check in staff needs to verify that you are legally able to enter country B since that's the end of your journey as far as they are concerned. They may not accept your onward ticket and they may not be able to verify whether you can enter country C if they don't fly there. If they deliver you to B and you are not admitted or sent back, the airline risks a heavy fine, so they tend to be conservative.
With two PNRs I strongly recommend getting a transit visa. It's not impossible to do without, but it requires careful research, many conditions must be met and you need to validate this with the first airline before check in.