Multiple cards with Barclays is fine. I think they do have exposure limits which do change over time based on your score and usage. I had the Cash Rewards card when I applied for the SM card. I got a $9600 limit on it when at the same time my limit on the Cash card was $1000. I called the underwriters and asked if they could increase my limit to 10K and during the conversation it appeared they could have increased it to 12K or 15K. I didn't take it, but did get them to also increase my Cash card to 2K. Since then my SM card has increased to 14K, which means that the two combined limits is higher that the total they were offering at the time of the SM app.
As far as PC'ing to the Apple card, that might not be an offer. Some banks won't allow PCs to cobranded or from cobranded cards, except sometimes to another cobranded card.
If you have been with Barclays over a year then it might be a good time to simply apply for the Apple card. I am not sure of the rewards structure on the card, but the Cash Rewards card might actually pay out more than the points with the Apple card. I think the biggest benefit to the Apple card is simply extended financing at 0% if I am correct?
It is by will alone I set my mind in motion.....
Always follow these rules: Only take a HP for a new account. Always use the best rewards card for that reward category. Don't close a card unless you know you really should. Never use more than 35% of a credit limit. Recon as much and as best you can. Use the introductory period to the best advantage. Get the signup bonus. Whenever possible PIF or balance transfer so you pay less in interest. Never give an excellent rating when it is actually the norm. Always look for a discount as more is always better.
Always accept candy from strangers because they have the best candy or from people you know have good candy.
Current (8/2015): Equifax 688, Transunion 689, Experian 708
