I have two defendants charged with murder that will be tried in two separate trials. Can defendant #2, whose case has not been tried yet, testify in the trial of defendant #1 and then claim the 5th when the State starts cross-examining her? The two defendants and the victim were related. I think that defendant #2 would like to testify about the prior relationship between the parties. Also, there are self defense and defense of third person issues in the trials. Of course, the co-defendant is the "third person." As always, any insight would be appreciated.

You can ask to do the direct and cross first in voir dire and if the witness refuses to submit to cross, the trial court can bar his testimony. Keller v. State, 662 S.W.2d 362 (Tex.Crim. App. 1984); Denham v. Deeds, 954 F.2d 1501, 1504 (9th Cir. 1992) (trial court did not err in refusing to allow a defense witness to testify where that witness, in a “dry run” out of the presence of the jury, refused on cross-examination to respond to questions as to noncollateral matters); U.S. v. Colon-Miranda, 992 F.Supp. 86, 89-90 (D.Puerto Rico,1998) ("Therefore, we cannot permit a witness to testify on direct if we have adequate reason to believe that he will invoke the Fifth Amendment on cross-examination regarding questions properly probing his credibility and bias."); Com. v. Drumgold, 668 N.E.2d 300, 313-14 (Mass. 1996).

cf. Soria v. State, 933 S.W.2d 46, 52-56 (Tex. Crim. App. 1996) (trial court limit defense psy. expert to hypotheticals where defendant refused to cooperate with exam by State expert); but cf. Stephens v. State, 59 S.W.3d 377 (Tex.App. -Houston [1 Dist. 2001, pet. ref'd) (where witness waived 5th amend., trial court should have compelled testimony rather than stricken direct testimony).
Not sure I understand Stephens but it doesn't seem right or workable.