Same actors; similar level of sweetness
Watch if you're into cute romantic fluff with supportive side characters and relatively little scheming second leads!

Both female leads end up an evil extra in the stories they fell into.
Both dramas make fun of common drama tropes and are hilarious to watch.
Plus Tiger and Rose had this one line that can help calm everyone who suffered from Second Male Lead Syndrome in Lost Romance:
"The male lead belongs to the female lead. The second male lead belongs to everyone (audience)."

While Xiao En did not write the novel she ended up in (and hence does not know how it will end), Xiaoqian in Tiger and Rose falls into a drama script she wrote herself, which means her take on how to change her fate would take a different turn than Lost Romance.

Without spoiling the ending -- watch until the end, you'll find parallels in how the male leads are set up too XD

Both works make fun about the current drama tropes and are extra meta to watch.
There's also a blend of the fictional and the real worlds in both dramas -- one character in Be Melodramatic is a drama scriptwriter and it's clear her drama is loosely based on her own story.

Female lead finds herself an extra in a fictional world and fights to become the female lead of her own story.
They also make fun of a lot of drama tropes so the comedic factor is high.