According to the author's FAQ, it's mostly compatible. Since you both have different love interests (and I assume you both aren't trying to woo the other guy), I think it should still be okay and events shouldn't get overwritten. There may still be some glitches with festivals, but for the most part, the game should work :)
From the FAQ of "Maggs Immersive Sam (X)Spicy or Sweet v2":
Q Is this mod compatible with Magg's Immersive Sebastian?
A Technically mostly, the mods will not conflict but there might be a few minor glitches that are not correctable due to the high level of immersion in the mods. Note the mods were not written to be played on the same save. The mod that loads last into Content Patcher (currently Immersive Sebastian) will take precedence on dialogue so Sam's dialogue will be pulled from Immersive Seb for any dialogue keys that are the same, including festival dialogue. I cannot fix this. All events will play on both mods, Note the wedding reception event will have glitches and both receptions will play for the first character you marry (may be fixed in the next update). You will see some minor event repeats since the mods share some cutscenes. Dialogue is written for an exclusive relationship with the farmer on both mods. This will not be modified. Some dialogue will be similar for each character, Sam's mod was originally based on Sebastian's mod outline.
