You could certainly do that; however, if you're using Cart buttons, your PayPal buttons would be adding items to one cart while your E-junkie buttons would add items to a separate cart, which is likely to confuse buyers.
That would not be a problem if you're using Buy Now buttons, which bypass any cart to take buyers directly to instant checkout for just one item at a time; however, E-junkie Buy Now buttons will only work with certain discount code settings, and you'd need to use a special version of the Buy Now button code that includes a field where buyers could enter their code before they click Buy Now -- see further details here:
http://www.e-junkie.com/ej/faq.discounts.buy-now.htm
