Hello, Blue_Delta_47, and welcome to the Grammar Exchange!
Blue_Delta_47 posted:I’m wondering whether it looks well-written and, most importantly, grammatically correct to use 2 synonyms at once in my following sentence:If it looks well-written and is grammatically correct when the audience read it, do you recommend me using a ‘comma’ after ‘muscle cells’ to prevent it from sounding too long for a sentence?Pacemaker Cells make up only about 1% of the total number of cardiac muscle cells and constitute the cardiac conduction system.
Your sentence is grammatically correct. There is no rule barring the use of synonymous verbs in a sentence with two coordinated verb phrases. The comma is optional: the sentence is correct with or without it.
What I think would improve the sentence from a stylistic standpoint is making one idea the main idea and the other the background idea, by using a nonrestrictive relative clause:
(A) Pacemaker Cells, which make up only about 1% of the total number of cardiac muscle cells, constitute the cardiac conduction system.
(B) Pacemaker Cells, which constitute the cardiac conduction system, make up only about 1% of the total number of cardiac muscle cells.
Are you sure you want to capitalize "Cells"?