Dual Port DDR SDRAM

I've never came across it. Given the principles of DRAM I doubt it exists. What is your bandwidth requirement?

Highly unlikely, there used to be a chip that would turn a standard dram into dual port, went up to 1Mbyte IIRC. At least 10 years ago so probably long obsolete.

This is being done quite a lot on graphic cards, where the graphic chipset itself (or in some cases a programmable gate array), is used to provide such access. The problem of course is that most DDR memories are 'burst' orientated devices, so the external buffer needs to be large enough to support a full burst. Also the complexity is very different for a sequential burst orientated read access (as needed on a video card), versus a generic read. A lot will depend on what speeds are needed, and the nature of the expected I/O. Some video DRAM's, offer the ability to have two pages (or more) open at once, to simulate dual port operations. Cypress do an interconnect family, to allow dual port access on a variety of memories. A search for 'FullFlex' devices, may provide what is needed.

Best Wishes

How about doing the old fashioned way with interleaved memory? Vital Industries did the a long time ago with slow 1K * 4 DRAM in their Squeeze Zoom digital video effects generator for live manipulation of broadcast quality video. By careful timing and using two pages of video ram it allowed one page to be updated while the second page was being displayed live.

It was the first effective DVE system on the US market, but apparently long gone from any broadcast use because of the cost to keep it hot and running.