I have a fairly comprehensive range of writing styles at my disposal, and I am frequently asked
(a) a method for learning a new hand and (b) how to avoid mixing them up in general usage.
It's possible to learn two different scripts at the same time, but it's best to avoid styles with some similarities, either in the lettering itself or in the tools used. For example, I would never advocate studying Copperplate and Spencerian, simultaneously. as the result could be unnecessarily confusing.
First of all, I should mention that I am entirely self-taught in all the scripts I use. I strongly believe in structured learning and in order to reach a reasonable standard, I had to devise a strict, self-learning method which works - so here goes.....
If I want to add a new script into my repertoire, I copy out the entire lowercase alphabet as closely as I can. I then study it in detail. Some of the letters may be fine straight off. I put them to one side, mentally, and write out the remainder again and again, each time missing out those which are OK. In this way, I may end up with a single letter which is still giving me trouble. I then spend however long it takes, writing only that letter over and over again, until it's right. If necessary, I enlarge individual letters by photocopying, to see where I'm going wrong. The end result is a new lowercase script at my disposal.....I then repeat the entire process with the uppercase letters. Once I have the whole alphabet down, I then move on to combinations of letters and finally, words. This can all take some time.
There is no point in repeatedly writing out only the easy bits - I concentrate on the hard bits until it all falls into place.
Avoid the strong temptation to move onto words until all the letters look right.
I knows that this may seem tedious, but very little that's worthwhile, comes easy!
It's better to keep the new style separate from the old ones, until it's fully formed in the mind, otherwise you can end up with a jumbled mess.
I firmly believe that self-taught, concentrated study in depth, works. I know that studying with a good teacher works well for most students, but it isn't always easy to recognize one - especially when starting out when everyone's 'calligraphy' looks great. As the quality of teaching seems to be very patchy, if possible, personal recommendation is safest. However, if a good teacher isn't available, well-structured, self-learning can be a perfectly adequate alternative. It's really just a matter of self-discipline.
I never have a tendency to mix styles inadvertently. This isn't because I'm particularly clever (which I'm not!) but because I've learned each script, individually in such depth, that they've lodged firmly and separately, in my memory. If I'm writing in a style which I don't often use, I sometimes have an exemplar at my side, just it case - but I rarely need it.
Adapting a style of lettering to suit your own personality is fine, but in my experience, it always works best if the groundwork has been done properly.
So there you have it. Nothing innovative and no 'magic wand' but an obvious system which works, with end results which can give great personal satisfaction.
Ken