They need separating and potting up in their own individual tubs - you can't maintain them together for long like you describe. De-pot them and divide the root mass as best you can with a bread knife - inevitably there will be some damage, but you've got no choice really. So opt for surgery now. Re-pot them in generous new tubs - especially the Cyathea, which is by far the faster grower of the two. The Blechnum will appreciate a lime-free ericaceous compost, whereas the Cyathea will be less fussy and be fine in any rich mixture. Maintain dampness and they should both get going again soon.