Trim Mark problem. CS5 Any suggestions?

Question

I am setting out A4 front and back covers as one piece of (A3+bleed) artwork to be reproduced by digital printing. After printing, the covers will be trimmed on four sides then wire bound.
I set up artwork with covers side by side with 10mm in centre to allow 5mm trim for each spine. My problem is that Illustrator CS5 adds trims on the proof which do not include a facility to specify centrefold trims or even a centreline. AI assumes all artwork will be an single rectangle and only puts trims on the four corners.
I can create two covers as two separate pieces of artwork with suitable bleed and trims, but then I must proof front and back separately and the printer must re-position each cover side by side arranging trims with his pre-press software. That is inefficient.
I simply want to hand over a facsimile proof and a complete .pdf fiIe with correct trims. I tried drawing the spine trims I required, but AI ignored them for proofing, even with the desk top enlarged to accommodate the trim marks. The Print dialogue box trims take priority. Has Adobe missed a trick here or am I overlooking a set-up requirement?
I considered taking the artwork into InDesign but ID also treats my artwork as one page with or without a centrefold, according to how I specify the pages.
But for the fold facility, the ID Print dialogue behaves the same as AI, as you might expect.
Can anyone offer suggestions other than the separate artwork option I have described?

Answer 1

Dear TerraAustralis,
I work in CS5 and I am trying to create trim marks so that I can manually adjust it after and create more of them since I have a multiple itmes to trim.
I get an error message when I try to create the mark:
I have a rectangle with or with stroke or fill (it does the same error), I selct Object-Trim marks
and I get this: <The filter cannot complete because of an unknown error>
NESS
what can I do to be able to edit the marks without making them all manually like before...
since the Crop marks command does not allow us to manipulate the marks?
Thank you in advance for your help.
cc