what method to encourage two flower spikes

No they don't - only the expensive ones! Sorry, I don't know why they do this, I have about 12 now, and sometimes the stems branch, sometimes not. Sometimes they produce two stems, more often one. They are all treated the same! Strange, isn't it!

You are doing well Sprizthenry! I've only got up to five so far. I suppose commercially produced ones are given extra light to produce more stalks. I have some I bought with two stalks but they've only got one now and only one has two. But I agree the cheaper ones have fewer!

Celandine, I'd like more - but there's no more room on the two sills where they live!