diff --git a/devito/ir/support/guards.py b/devito/ir/support/guards.py
index deba5be148..dbb1fe9ca7 100644
--- a/devito/ir/support/guards.py
+++ b/devito/ir/support/guards.py
@@ -300,6 +300,7 @@ def xandg(self, d, guard):
 
     def pairwise_or(self, d, *guards):
         m = dict(self)
+        guards = list(guards)
 
         if d in m:
             guards.append(m[d])
diff --git a/devito/passes/clusters/buffering.py b/devito/passes/clusters/buffering.py
index 61676d2e7f..2a8c78e7fc 100644
--- a/devito/passes/clusters/buffering.py
+++ b/devito/passes/clusters/buffering.py
@@ -182,10 +182,10 @@ def callback(self, clusters, prefix):
             for _, v in descriptors.items():
                 if not v.is_readonly:
                     continue
-                if c is not v.firstread:
+                if c not in v.firstread:
                     continue
 
-                idxf = v.last_idx
+                idxf = v.last_idx[c]
                 idxb = mds[(v.xd, idxf)]
 
                 lhs = v.b.indexify()._subs(v.xd, idxb)
@@ -225,10 +225,10 @@ def callback(self, clusters, prefix):
             for _, v in descriptors.items():
                 if v.is_readonly:
                     continue
-                if c is not v.lastwrite:
+                if c not in v.lastwrite:
                     continue
 
-                idxf = v.last_idx
+                idxf = v.last_idx[c]
                 idxb = mds[(v.xd, idxf)]
 
                 lhs = v.f.indexify()._subs(v.dim, idxf)
@@ -508,17 +508,19 @@ def subdims_mapper(self):
 
     @cached_property
     def firstread(self):
+        mapper = {}
         for c in self.clusters:
             if c.scope.reads.get(self.f):
-                return c
-        return None
+                mapper.setdefault(c.guards, c)
+        return tuple(mapper.values())
 
     @cached_property
     def lastwrite(self):
+        mapper = {}
         for c in reversed(self.clusters):
             if c.scope.writes.get(self.f):
-                return c
-        return None
+                mapper.setdefault(c.guards, c)
+        return tuple(mapper.values())
 
     @property
     def is_read(self):
@@ -529,7 +531,7 @@ def is_read(self):
 
     @property
     def is_write(self):
-        return self.lastwrite is not None
+        return bool(self.lastwrite)
 
     @property
     def is_readonly(self):
@@ -604,8 +606,14 @@ def last_idx(self):
             * `time-1` in the case of `foo(u[time-1], u[time], u[time+1])`
                with a backwards-propagating `time` Dimension.
         """
+        mapper = {}
         func = vmax if self.is_forward_buffering else vmin
-        return func(*[Vector(i) for i in self.indices])[0]
+        for c in self.lastwrite + self.firstread:
+            indices = extract_indices(self.f, self.dim, [c])
+            idx = func(*[Vector(i) for i in indices])[0]
+            mapper[c] = idx
+
+        return frozendict(mapper)
 
     @cached_property
     def first_idx(self):
diff --git a/devito/types/parallel.py b/devito/types/parallel.py
index 891eaad176..ff9e1405a9 100644
--- a/devito/types/parallel.py
+++ b/devito/types/parallel.py
@@ -14,6 +14,7 @@
 
 from devito.exceptions import InvalidArgument
 from devito.parameters import configuration
+from devito.symbolics import search
 from devito.tools import as_list, as_tuple, is_integer
 from devito.types.array import Array, ArrayObject
 from devito.types.basic import Scalar, Symbol
@@ -75,8 +76,8 @@ def _arg_defaults(self, **kwargs):
             raise InvalidArgument("Cannot determine `npthreads`") from None
 
         # If a symbolic object, it must be resolved
-        if isinstance(npthreads, NPThreads):
-            npthreads = kwargs.get(npthreads.name, npthreads.size)
+        for th in search(npthreads, NPThreads):
+            npthreads = npthreads._subs(th, kwargs.get(th.name, th.size))
 
         return {self.name: max(base_nthreads - npthreads, 1)}