Does a cruising dinghy need another dinghy to act as a tender?
It is not essential to carry a tender since most dinghies are small and light enough to act as their own tender, being brought up to a landing point whenever the crew need to get ashore. However, there are some situations where a small tender can be quite useful, particularly with a heavy cruising dinghy. If a heavy dinghy is allowed to dry out on a beach as the tide ebbs the crew may not easily be able to get it back into the water until the tide returns, which may be many hours. Thus if you want to make a quick trip ashore it may be preferable to anchor or moor the larger boat in deep water and go ashore in a tender. A small tender that you can carry around on shore is also useful when the only landing point is likely to damage a boat, e.g. there may be rocks, submerged obstacles, fishing boats going to and fro etc. In this situation you can anchor off or take a mooring then go ashore in the tender and find somewhere to safely leave the tender well clear of the water.
I have found that one of the tiny inflatable boats sold as a beach toy makes a suitable tender for a cruising dinghy. I have had several of these beach toy tenders over the years. Even if you use them carefully they tend to develop leaks but they only cost a few pounds to replace whereas a proper yacht tender costs hundreds of pounds and would in any case be too bulky to carry on board most dinghies. Toy inflatables are usually poor rowing boats since apart from anything else the rowlocks are too weak and flexible. I do not use oars with mine, I prefer a pair of home-made paddles which look very much like ping pong bats. The picture below shows my tender in use with the small paddles on the Odet River in Southern Brittany. This particular toy inflateble is an 'Octopus 110' which is large enough to take two adults. I think the next size down is the 'Octopus 90' which is adequate for one adult. The Octopus range of toy inflatables is just a bit upmarket from the really cheap ones. In case of a puncture there is a double main air chamber but I am not sure how much of a safety feature that is since you would not have a useable boat left once once the largest chamber is punctured. The largest air chamber features a proper valve with a screw on lid. Unfortunately the second largest chamber has only a small plastic air connection with a moulded plastic flap as a valve, as on the really cheap toy boats, this is much less satisfactory for easy inflation.
Get a good pump to use with your inflatable, otherwise it will take ages to inflate. If a pump is supplied with a cheap toy inflatable this pump may well be poor quality. Get an inflatable large enough to feel secure but no larger than necessary otherwise again you will be all day inflating it. Remember that the inflatable only needs to carry the one crew member who has stayed on board to anchor or moor the larger boat, any other crew can usually be put ashore first. Do not leave loose items aboard your inflatable while it is moored by its painter - a gust of wind can capsize it and all is then lost.