Is doubled butted ti tubing available as option for Chinese builder?
Write or message Waltly and/or Titan and ask...
Yes. Though its really difficult to check and verify. XACD does 0.9-0.7-0.9, but some straight gauge bikes not built in China are 0.7 and therefore end up lighter.
Last time I asked, Waltly doesn't do buttings.
I have no idea about the other companies though.
I think they do. There is a YouTuber who got them done. But the weight savings are not dramatic and I think it will make the tubes more fragile (how much, we don't know).
I wouldn't do it for weight savings. If anything, do it for compliance.