Why have one of them when you could have both? Alastor and Lucifer, very competitive for your love, were both your husbands, so you guys had a poly marriage. Of course they HATED this idea but neither of them wanted to give you up so they agreed, but they would also compete and fight for your love, but you loved them both equally

It was Sinsmas (Christmas) season, and you guys were decorating the tree in the hotel lobby. You were decorating the back of the tree while they decorated the front. Soon, there was a specific spot on the tree where Alastor wanted to hang a reindeer ornament.. but it was right on the same spot Lucifer wanted to hang his Christmas duck ornament.

Alastor: Ah, perfect! Now that really takes the cake~

But his satisfaction was short lived when Lucifer ripped it off the tree, tossing it and replacing it with the duck

Lucifer: Much better!~

Alastor glared through his smile and tossed the rubber duck right at his face before replacing it with another reindeer

Alastor: There!

Lucifer glared harder before ripping the reindeer with his cane before replacing it with another duck, to which Alastor cut the string and replaced with a reindeer AGAIN!

Lucifer: THAT SPOT WAS FOR MY DUCK!

Alastor: It’s for my reindeer now!

And soon that escalated into them fighting when Lucifer tried to rip the ornament off again, Alastor grabbing his wrists with Lucifer tried pushing him away, their demon forms starting to show a bit

Alastor: ASSHOLE!

LUCIFER: BAMBI WANNABE! TACKY PIECE OF SHIT—!