There are tricks to the uni to uni..tie in fig 8 manner not uni hahaha..maintain tension on tag ends when cinching down, but dont pull them so the knot cinches down smoothly, lube the knot and line properly with spit before this, help it cinch and shape right, the fig 8's have a particular shape to them when they cinch right with the loops getting tighter/narrower towards the line/tag, like a slight pyramid/triangular shape I don't know how to describe it otherwise.. With the doubled mainline you must take care to cinch the extra line and tag down evenly with the main line, it all makes sense when you try the knot..Make sure there are no burns from slipping or pigtails on your line from cinching the knot down and do the wind the line 3 or 4 times over your hands and over your thumb and hands on knees and try break the knot with your leg strength and if you cant then a fish wont.
Once you have practiced it and are confident in it..the only reason you'd need another is when you need a smaller diameter or for some other practical reason, in terms of function, tied right this knot is perfectly functional and you don't really need another join for mono to mono..If the knot is on a light tackle setup, for bass or the likes I dont double the mainline, but the knot is weaker..If you don't cinch it down properly and it slips, it will break, the doubled line more prone to this, but I've caught many 200lb tuna and a couple 30lb plus yellowtails on the double mainline fig8 to fig8 leader, 4 turn to 3 turn...there aren't many fish in the sea that fight harder than those two so I have great trust in that knot. Was shown to me by an old timer after seeing me loose fish...He was like once you get this knot down, and the fig8 for terminal tackle and his variation on that, there aren't many other knots one needs for mono. Since that the number of knots I regularly use has gone down to less than a handful, including fg for braid etc..
You can get all fancy, but pylstert's given you the gold..and when the fancy stuff gets bust off when fishing off the rocks, or on the boat, you need to know the fig8 to fig8/ uni to uni anyway, and have it down, as you need to tie something strong quick before the bite goes off...and you arent going to be sitting there on the beach or on a rocking boat threading needles and stuff..