documentclass{article}
 usepackage[latin9]{inputenc}
 %usepackage{soul}
 usepackage{ulem}

 usepackage[style=authoryear-comp,natbib=true,%
 backend=biber]{biblatex}

 usepackage{filecontents}
 begin{filecontents}{jobname.bib}
 @Book{Darwin.84,
   author =      {C. D"{a}rwin},
   title =   {The different forms of flowers on plants of the same species},
   publisher =   {John Murray},
   address =      {London},
   edition =      2,
   year =    1884}

 @ARTICLE{Davies.pnas04,
 AUTHOR = {Davies, T. J. AND Barraclough, T. G. AND Chase, M. W. AND Soltis, P. S. AND Soltis, D. E. AND D"{a}rwin, C.},
 TITLE = {D"{a}rwin's abominable mystery: {Insights} from a supertree of the angiosperms.},
 JOURNAL = {Proc. Natl. Acad. Sci. U.S.A.},
 VOLUME = {101},
 PAGES = {1904--1909},
 YEAR = {2004},
 MONTH = Feb,
 NUMBER = {7}
 }

 @ARTICLE{Vamosi.el10,
 AUTHOR = {Vamosi, J. C. AND Vamosi, S. M.},
 TITLE = {Key innovations within a geographical context in flowering plants: towards resolving {D"{a}rwin}'s abominable mystery.},
 JOURNAL = {Ecol. Lett.},
 VOLUME = {13},
 PAGES = {1270--1279},
 YEAR = {2010},
 MONTH = Oct,
 NUMBER = {10}
 }

 end{filecontents}
 addbibresource{jobname.bib}


 makeatother

 begin{document}


 citet{Davies.pnas04,Vamosi.el10} cite uline{citet{Darwin.84}}.

 %citet{Davies.pnas04,Vamosi.el10} cite ul{citet{Darwin.84}}.

 printbibliography

 end{document}

documentclass{article}

usepackage[style=authoryear-comp, backend=biber]{biblatex}
addbibresource{biblatex-examples.bib}

usepackage{kantlipsum}
usepackage[normalem]{ulem}
usepackage{soul}

newattribute{ulattr}

usepackage{luacode}
begin{luacode}
local HLIST = node.id("hlist")
local RULE = node.id("rule")
local GLUE = node.id("glue")
local KERN = node.id("kern")
local ULATTR = luatexbase.attributes["ulattr"]

function get_lines(head)
  for line in node.traverse_id(HLIST, head) do
    underline(line.list, line.glue_order,
      line.glue_set, line.glue_sign)
  end
  luatexbase.remove_from_callback("post_linebreak_filter",
                                  "get_lines")
  return head
end

local function good_item(item)
  if item.id == GLUE and
    (item.subtype == node.subtype("leftskip")
     or item.subtype == node.subtype("rightskip")
     or item.subtype == node.subtype("parfillskip")) then
      return false
  else
    return true
  end
end

function underline(head, order, ratio, sign)
  local item = head
  while item do
    if node.has_attribute(item, ULATTR)
      and good_item(item) then
        local item_line = node.new(RULE)
        item_line.depth = tex.sp("1.4pt")
        item_line.height = tex.sp("-1pt")
        local end_node = item
        while end_node.next and
          good_item(end_node.next) and
          node.has_attribute(end_node.next, ULATTR) do
            end_node = end_node.next
        end
        item_line.width = node.dimensions
          (ratio, sign, order, item, end_node.next)
        local item_kern = node.new(KERN, 1)
        item_kern.kern = -item_line.width
        node.insert_after(head, end_node,
                          item_kern)
        node.insert_after(head, item_kern,
                          item_line)
        item = end_node.next
    else
      item = item.next
    end
  end
end
end{luacode}

newcommand{luaunderline}[1]{%
  quitvmode
  setattribute{ulattr}{1}%
  #1%
  unsetattribute{ulattr}%
  directlua{%
    luatexbase.add_to_callback("post_linebreak_filter",
                               get_lines, "get_lines")}%
}

newcommand{luaunderlineswitch}{%
  quitvmodesetattribute{ulattr}{1}%
  directlua{%
    luatexbase.add_to_callback("post_linebreak_filter",
                               get_lines, "get_lines")}%
}

begin{document}
  textcite{sigfridsson,nussbaum} cite underline{textcite{sigfridsson}}.

  textcite{sigfridsson,nussbaum} cite uline{textcite{sigfridsson}}.

%  textcite{sigfridsson,nussbaum} cite ul{textcite{sigfridsson}}.% <- gives a error

  textcite{sigfridsson,nussbaum} cite luaunderline{textcite{sigfridsson}}.

  kant[1]
end{document}