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Classical T Tauri stars are bright in the soft X-ray and far UV bands, because of large amounts of plasma at T~10⁵-10⁶ K, associated with the accretion-shock regions. Inspecting the emission from the shock region is important since it can potentially reveal fundamental properties of the accretion-stream material (i.e. geometry/density/velocity/abundances). However, the precise location of these hot plasmas (pre-shock? post-shock? different shock regions with different temperatures?) is still unclear. To constrain location and properties of these accretion-related hot plasmas, X-rays and UV observations are needed. The next XMM call (deadline 6 October 2017) offers the possibility to investigate this issue. I would like to discuss with you the opportunity to propose a joint XMM+HST program, focused on TW Hya, to perform time-resolved high-resolution spectroscopy on time scales down to 15 ks, simultaneously in the X-ray and UV bands. Correlated or uncorrelated variability of plasmas at 10⁵ and 10⁶ K will indicate whether or not they are located in the same accretion-shock regions. That will provide important constraints on the physical properties of the accretion streams in CTTS.