QSJN 4 UKR asks, "Why did the Unicode Consortium think that combination of one base character and few combining is possible, and combination of few base characters with one combining character is not? E.g. U+0483 "tilda" has to cover a number. Whole number!"
For mathematical constructs in general, you need to use mathematical typography. You can see one way to support this in nearly plain text in Section 3.10 of Unicode Technical Note #28 (http://www.unicode.org/notes/tn28/UTN28-PlainTextMath-v3.pdf). To place a tilde over a couple of base characters, you can use the wide accents encoded in Unicode as Karl Pentzlin pointed out. And මේ අකුරු ලියා ඇත්තේ යුනිකෝඩ් අකුරෙනි looks like Sinhala text to me. Murray
