Tandem feeding prioritises the new baby, so you still get colostrum after baby is born. Older child can drink the colostrum and the body makes more, just as if you were feeding multiple babies. Older child is mainly getting immune factors and comfort from breastfeeding (depending on age, eating habits and overall health of course), so tandem feeding is fine for them too, just the milk is tailored by the body for younger child.
Different people have different experiences with regard to pregnancy. Firstly, you obviously need to be ovulating to fall pregnant and breastfeeding can inhibit this, so some people do stop breastfeeding in order to actually fall pregnant (and we had issues with fertility for first child, doctor wouldn't allow any fertility drugs whilst breastfeeding). I did fall pregnant twice whilst breastfeeding, but if you're wanting a close gap that may be an issue.
Once pregnant, most people find their supply drops, often to the point that the older child weans. Usually by then they are eating foods, and depending on age may not need milk as their primary food source. If it is, baby will cluster feed and supply should re-establish. I certainly had less milk whilst pregnant, but mine were all over 1 when I fell pregnant and were eating solids very well, so whilst there was still a benefit to having breastmilk, it wasn't their sole nutrition. They were not interested in stopping breastfeeding, hence my journey into tandem feeding.
I didn't have any problem with soreness and feeding during pregnancy, I guess if you do nipple shields or being strict on attachment/position might be needed to help.
I did have problem with soreness after the baby was born, I think we'd cut down feeds for older child to maybe twice a day (mine were about 2 when the next baby was born). But then newborn started feeding every 2-3 hours, which is a shock to the nipples, and obviously they are learning to feed, so attachment can be wrong etc, causing damage. Older child not wanting to miss out and feeling somewhat jealous also wants more feeds. And when the milk "come in" toddler thinks it is amazing, and newborn doesn't get drowned in milk! But things settled quite quickly into a routine.
Breasts are amazing, they mimic a pattern of feeds, so once we were settled (maybe 2 weeks in) it all worked seamlessly, I was fortunate to have good supply and everyone seemed content after a feed, so felt there was enough.