Zoro and sanji are characters that have been around for a long time. An interesting take here is that their relationship is actually a parallel to crewmates in roger pirates.
Zoro is the Shanks of the strawhat pirates, while sanji is the Buggy of the strawhat pirates. Let's just look at the evidences.
Zoro and sanji often fight with each other. Just like shanks and buggy.
Now, let's take a closer look for each pair (zoro-shanks, sanji-buggy)
Zoro and shanks. Both are swordsman with scars on their left eye, and have similar pose in their bounty poster (looking at their left side). Both also have fought kaido and mihawk.
Meanwhile...
Both sanji and buggy are characters with lots of gag scenes.
As closing words, this thread specifically portrays the parallel relationship between zoro-sanji with shanks-buggy. The characters can also be seen in another parallel when looked at individually (for instances zoro is rayleigh of SHP and luffy is roger of SHP, but this is already explained by
@hedera__helix in his thread)