Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
Joint gifts made in the way you describe will be split on a 50:50 basis unless the monies in the joint account aren't owned equally. You need to keep a record of gifts made so that your executors know what was gifted in the seven years before your passing. You should specify in that record if a joint gift was made on a 50:50 basis or some other basis. If the gift of £50,000 was split equally between you and either you or your wife died within seven years of making it, then £25,000 would use up part of that individual's £325,000 nil-rate IHT band. The nil-rate band is used first against gifts made in the seven years before death in chronological order. Taper relief which you can read about here reduces the IHT charge after three years.
I hope this helps but let me know if you have any further questions.