It's not recommended, but both should have not much different timings and required voltages at this speed, so I guess it will be fine. The motherboard typically takes timings from the first memory module and sets it for all banks so you may switch memory sticks in slots to check if Samsung or Hynix in the first slots runs better.
A/B/C/D/E/F/... die is just a "street" naming. The most popular commercial series are B and C for Samsung, B or E for Micron, and A or M for Hynix. Sometimes a die letter suggests what is used in a specific generation like Samsung B or Micron B is on the mass market only in a specific design/capacity. For Hynix, M-die naming was used multiple times in DDR4 and current DDR5 are also M.
The same die letters in various IC brands are not related. For example, Samsung B is totally different than Micron B, with different timings and capacity/density.
For a typical user (or maybe those who care or need it for something), a manufacturer and a die letter are enough info for almost everything.
If you are mixing IC then it doesn't matter what die is there. It's recommended to use the same IC density, but in most cases, you don't really know what IC is in a memory kit. Most manufacturers are using everything that matches the XMP profile and in theory, if you buy two memory kits under the same product number but different IC then they still should work together. In reality, it's not so obvious, but at least at lower frequencies, it should work. By lower frequencies, I assume everything like 3600 or less.