it is absolutely not necessary to have two, but it depends on what size negs you;re wanting to print as to what enlarger you get. they all have a maximum size, but most should cover sizes smaller than that.
Different lenses cover different neg sizes - the convention being 35mm is around 50mm lens, 6x6 is around 80mm lens, 6x7-6x9 105mm lens and 5x4 is a 150mm lens. When I only printed 6x6 and 35mm an 80mm did fine for both, although of course I could have got larger prints from my 35mm negs with a 50mm lens.
One thing to consider is if you're wanting a diffusion or a condensor enlarger. Some of the condenser types need supplementary condenser lenses or different lens boards to enable 35mm to be used - best check it out before you buy to make sure all that you need is present - particularly if you're buying used.
One other thing - it's best to get one that allows you to adjust the negative plane and lens plane levels to ensure they are aligned.
I dont think you can go wrong with the Meopta range as a first enlarger. they're easily adjustable and easy to use, and come in both diffuse and condensing light types. I used a Meopta Opemus before I got my devere and it was perfect for 35mm and 6x6.
